broadband connection establishes then drop after a few seconds

When connecting a cell phone to share the cell network connection, the link gets established then drops after a few seconds. Doing the same on Trusty works correctly.

Here is the end of a successful connection on Trusty :
==========================================
Jun 5 14:13:54 trusty NetworkManager[850]: <info> Activation (usb0) successful, device activated.
Jun 5 14:13:54 trusty dbus[756]: [system] Activating service name='org.freedesktop.nm_dispatcher' (using servicehelper)
Jun 5 14:13:54 trusty dbus[756]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Jun 5 14:14:05 trusty ModemManager[769]: <info> Creating modem with plugin 'Generic' and '2' ports
Jun 5 14:14:05 trusty ModemManager[769]: <info> Modem for device at '/sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.4' successfully created
Jun 5 14:14:09 trusty ModemManager[769]: <warn> couldn't load Manufacturer: 'Serial command timed out'
Jun 5 14:14:11 trusty NetworkManager[850]: <info> (usb0): IP6 addrconf timed out or failed.
Jun 5 14:14:11 trusty NetworkManager[850]: <info> Activation (usb0) Stage 4 of 5 (IPv6 Configure Timeout) scheduled...
Jun 5 14:14:11 trusty NetworkManager[850]: <info> Activation (usb0) Stage 4 of 5 (IPv6 Configure Timeout) started...
Jun 5 14:14:11 trusty NetworkManager[850]: <info> Activation (usb0) Stage 4 of 5 (IPv6 Configure Timeout) complete.
Jun 5 14:14:12 trusty ModemManager[769]: <warn> couldn't load Model: 'Serial command timed out'
Jun 5 14:14:13 trusty ntpdate[2828]: no server suitable for synchronization found
Jun 5 14:14:15 trusty ModemManager[769]: <warn> couldn't load Revision: 'Serial command timed out'
Jun 5 14:14:18 trusty ModemManager[769]: <warn> couldn't load Equipment Identifier: 'Serial command timed out'
Jun 5 14:14:22 trusty ModemManager[769]: <warn> couldn't load IMEI: 'Serial command timed out'
Jun 5 14:14:22 trusty ModemManager[769]: <info> Modem: state changed (unknown -> disabled)
Jun 5 14:14:22 trusty NetworkManager[850]: <warn> (ttyACM0): failed to look up interface index
Jun 5 14:14:22 trusty NetworkManager[850]: <info> (ttyACM0): new Broadband device (driver: 'cdc_acm, cdc_ether' ifindex: 0)
Jun 5 14:14:22 trusty NetworkManager[850]: <info> (ttyACM0): exported as /org/freedesktop/NetworkManager/Devices/3
Jun 5 14:14:22 trusty NetworkManager[850]: <info> (ttyACM0): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
Jun 5 14:14:22 trusty NetworkManager[850]: <info> (ttyACM0): deactivating device (reason 'managed') [2]
Jun 5 14:14:22 trusty NetworkManager[850]: <info> (ttyACM0): device state change: unavailable -> disconnected (reason 'none') [20 30 0]
Jun 5 14:14:58 trusty wpa_supplicant[1063]: wlan0: CTRL-EVENT-SCAN-STARTED
Jun 5 14:15:02 trusty wpa_supplicant[1063]: nl80211: send_and_recv->nl_recvmsgs failed: -33

A connection using the same cell phone on Wily has this log :
================================================
Jun 5 14:20:30 wily NetworkManager[836]: <info> Activation (usb0) successful, device activated.
Jun 5 14:20:30 wily dbus[897]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service'
Jun 5 14:20:30 wily systemd[1]: Starting Network Manager Script Dispatcher Service...
Jun 5 14:20:30 wily NetworkManager[836]: <warn> dnsmasq appeared on DBus: :1.81
Jun 5 14:20:30 wily NetworkManager[836]: <info> Writing DNS information to /sbin/resolvconf
Jun 5 14:20:30 wily dnsmasq[2869]: configuration des serveurs amonts à partir de DBus
Jun 5 14:20:30 wily dnsmasq[2869]: utilise le serveur de nom 192.168.42.129#53
Jun 5 14:20:30 wily dbus[897]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Jun 5 14:20:30 wily systemd[1]: Started Network Manager Script Dispatcher Service.
Jun 5 14:20:30 wily nm-dispatcher: Dispatching action 'up' for usb0
Jun 5 14:20:30 wily NetworkManager[836]: status: Impossible de se connecter à Upstart: Failed to connect to socket /com/ubuntu/upstart: Connexion refusée
Jun 5 14:20:30 wily systemd[1]: Stopping LSB: Start NTP daemon...
Jun 5 14:20:30 wily ntp[3063]: * Stopping NTP server ntpd
Jun 5 14:20:30 wily ntpd[1045]: ntpd exiting on signal 15
Jun 5 14:20:30 wily ntp[3063]: ...done.
Jun 5 14:20:30 wily systemd[1]: Stopped LSB: Start NTP daemon.
Jun 5 14:20:30 wily systemd[1]: Reloaded OpenBSD Secure Shell server.
Jun 5 14:20:30 wily ntpdate[3075]: name server cannot be used: Temporary failure in name resolution (-3)
Jun 5 14:20:30 wily systemd[1]: Starting LSB: Start NTP daemon...
Jun 5 14:20:30 wily ntp[3138]: * Starting NTP server ntpd
Jun 5 14:20:30 wily ntpd[3149]: ntpd 4.2.6p5@1.2349-o Mon Apr 13 17:00:14 UTC 2015 (1)
Jun 5 14:20:30 wily ntpd[3158]: proto: precision = 0.171 usec
Jun 5 14:20:30 wily ntpd[3158]: ntp_io: estimated max descriptors: 1024, initial socket boundary: 16
Jun 5 14:20:30 wily ntpd[3158]: Listen and drop on 0 v4wildcard 0.0.0.0 UDP 123
Jun 5 14:20:30 wily ntp[3138]: ...done.
Jun 5 14:20:30 wily systemd[1]: Started LSB: Start NTP daemon.
Jun 5 14:20:30 wily ntpd[3158]: Listen and drop on 1 v6wildcard :: UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 2 lo 127.0.0.1 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 3 lxcbr0 10.0.3.1 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 4 virbr0 192.168.122.1 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 5 usb0 192.168.42.109 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 6 lo ::1 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 7 lxcbr0 fe80::4e2:4cff:fe0e:2360 UDP 123
Jun 5 14:20:30 wily ntpd[3158]: Listen normally on 8 usb0 fe80::8cf4:f3ff:fe50:cfdd UDP 123
Jun 5 14:20:30 wily ntpd[3158]: peers refreshed
Jun 5 14:20:30 wily ntpd[3158]: Listening on routing socket on fd #25 for interface updates
Jun 5 14:20:30 wily ntpd[3158]: Deferring DNS for 0.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3158]: Deferring DNS for 1.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3158]: Deferring DNS for 2.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3158]: Deferring DNS for 3.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3158]: Deferring DNS for ntp.ubuntu.com 1
Jun 5 14:20:30 wily ntpd[3183]: signal_no_reset: signal 17 had flags 4000000
Jun 5 14:20:30 wily systemd[1]: Stopping LSB: Start NTP daemon...
Jun 5 14:20:30 wily ntp[3216]: * Stopping NTP server ntpd
Jun 5 14:20:30 wily ntpd[3158]: ntpd exiting on signal 15
Jun 5 14:20:30 wily ntp[3216]: ...done.
Jun 5 14:20:30 wily systemd[1]: Stopped LSB: Start NTP daemon.
Jun 5 14:20:30 wily systemd[1]: Reloaded OpenBSD Secure Shell server.
Jun 5 14:20:30 wily ntpdate[3229]: name server cannot be used: Temporary failure in name resolution (-3)
Jun 5 14:20:30 wily systemd[1]: Starting LSB: Start NTP daemon...
Jun 5 14:20:30 wily ntp[3266]: * Starting NTP server ntpd
Jun 5 14:20:30 wily ntpd[3274]: ntpd 4.2.6p5@1.2349-o Mon Apr 13 17:00:14 UTC 2015 (1)
Jun 5 14:20:30 wily ntpd[3275]: proto: precision = 0.121 usec
Jun 5 14:20:30 wily ntpd[3275]: ntp_io: estimated max descriptors: 1024, initial socket boundary: 16
Jun 5 14:20:30 wily ntpd[3275]: Listen and drop on 0 v4wildcard 0.0.0.0 UDP 123
Jun 5 14:20:30 wily ntp[3266]: ...done.
Jun 5 14:20:30 wily systemd[1]: Started LSB: Start NTP daemon.
Jun 5 14:20:30 wily ntpd[3275]: Listen and drop on 1 v6wildcard :: UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 2 lo 127.0.0.1 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 3 lxcbr0 10.0.3.1 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 4 virbr0 192.168.122.1 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 5 usb0 192.168.42.109 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 6 lo ::1 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 7 lxcbr0 fe80::4e2:4cff:fe0e:2360 UDP 123
Jun 5 14:20:30 wily ntpd[3275]: Listen normally on 8 usb0 fe80::8cf4:f3ff:fe50:cfdd UDP 123
Jun 5 14:20:30 wily ntpd[3275]: peers refreshed
Jun 5 14:20:30 wily ntpd[3275]: Listening on routing socket on fd #25 for interface updates
Jun 5 14:20:30 wily ntpd[3275]: Deferring DNS for 0.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3275]: Deferring DNS for 1.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3275]: Deferring DNS for 2.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3275]: Deferring DNS for 3.ubuntu.pool.ntp.org 1
Jun 5 14:20:30 wily ntpd[3275]: Deferring DNS for ntp.ubuntu.com 1
Jun 5 14:20:30 wily ntpd[3277]: signal_no_reset: signal 17 had flags 4000000
Jun 5 14:20:32 wily ntpd_intres[3183]: parent died before we finished, exiting
Jun 5 14:20:32 wily ntpd_intres[3277]: DNS 0.ubuntu.pool.ntp.org -> 109.69.184.209
Jun 5 14:20:32 wily ntpd_intres[3277]: DNS 1.ubuntu.pool.ntp.org -> 91.121.169.20
Jun 5 14:20:32 wily ntpd_intres[3277]: DNS 2.ubuntu.pool.ntp.org -> 212.83.179.156
Jun 5 14:20:32 wily ntpd_intres[3277]: DNS 3.ubuntu.pool.ntp.org -> 212.43.246.10
Jun 5 14:20:32 wily ntpd_intres[3277]: DNS ntp.ubuntu.com -> 91.189.89.199
Jun 5 14:20:37 wily ntpd_intres[1048]: parent died before we finished, exiting
Jun 5 14:20:40 wily ModemManager[818]: <info> Creating modem with plugin 'Generic' and '2' ports
Jun 5 14:20:40 wily ModemManager[818]: <info> Modem for device at '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-4' successfully created
Jun 5 14:20:44 wily ModemManager[818]: <warn> couldn't load Manufacturer: 'Serial command timed out'
Jun 5 14:20:47 wily ModemManager[818]: <warn> couldn't load Model: 'Serial command timed out'
Jun 5 14:20:50 wily ModemManager[818]: <warn> couldn't load Revision: 'Serial command timed out'
Jun 5 14:20:53 wily ModemManager[818]: <warn> couldn't load Equipment Identifier: 'Serial command timed out'
Jun 5 14:20:57 wily ModemManager[818]: <warn> couldn't load IMEI: 'Serial command timed out'
Jun 5 14:20:57 wily ModemManager[818]: <info> Modem: state changed (unknown -> disabled)
Jun 5 14:20:57 wily NetworkManager[836]: <warn> (ttyACM0): failed to look up interface index
Jun 5 14:20:57 wily NetworkManager[836]: <info> (usb0): device state change: activated -> unmanaged (reason 'removed') [100 10 36]
Jun 5 14:20:57 wily NetworkManager[836]: <info> (usb0): deactivating device (reason 'removed') [36]
Jun 5 14:20:57 wily NetworkManager[836]: <info> (usb0): canceled DHCP transaction, DHCP client pid 2864
Jun 5 14:20:57 wily kernel: [ 658.934305] cdc_ether 1-4:1.3 usb0: kevent 12 may have been dropped
Jun 5 14:20:57 wily kernel: [ 658.934315] cdc_ether 1-4:1.3 usb0: kevent 12 may have been dropped
Jun 5 14:20:57 wily dnsmasq[2503]: reading /etc/resolv.conf
Jun 5 14:20:57 wily dnsmasq[2503]: using nameserver 127.0.1.1#53
Jun 5 14:20:57 wily NetworkManager[836]: _nl_get_vtable: assertion 'vtable.handle' failed
Jun 5 14:20:57 wily avahi-daemon[812]: Withdrawing address record for fe80::8cf4:f3ff:fe50:cfdd on usb0.
Jun 5 14:20:57 wily avahi-daemon[812]: Leaving mDNS multicast group on interface usb0.IPv6 with address fe80::8cf4:f3ff:fe50:cfdd.
Jun 5 14:20:57 wily avahi-daemon[812]: Interface usb0.IPv6 no longer relevant for mDNS.
Jun 5 14:20:57 wily avahi-daemon[812]: Withdrawing address record for 192.168.42.109 on usb0.
Jun 5 14:20:57 wily avahi-daemon[812]: Leaving mDNS multicast group on interface usb0.IPv4 with address 192.168.42.109.
Jun 5 14:20:57 wily avahi-daemon[812]: Interface usb0.IPv4 no longer relevant for mDNS.
Jun 5 14:20:57 wily dnsmasq[1559]: Lecture de /etc/resolv.conf
Jun 5 14:20:57 wily dnsmasq[1559]: utilise le serveur de nom 127.0.1.1#53
Jun 5 14:20:57 wily NetworkManager[836]: <info> Writing DNS information to /sbin/resolvconf
Jun 5 14:20:57 wily dnsmasq[2869]: configuration des serveurs amonts à partir de DBus
Jun 5 14:20:57 wily NetworkManager[836]: <info> NetworkManager state is now CONNECTED_LOCAL
Jun 5 14:20:57 wily NetworkManager[836]: <info> (usb0): link disconnected
Jun 5 14:20:57 wily NetworkManager[836]: <info> NetworkManager state is now DISCONNECTED
Jun 5 14:20:57 wily kernel: [ 658.950918] IPv6: ADDRCONF(NETDEV_UP): usb0: link is not ready
Jun 5 14:20:57 wily dbus[897]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service'
Jun 5 14:20:57 wily systemd[1]: Starting Network Manager Script Dispatcher Service...
Jun 5 14:20:57 wily dbus[897]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Jun 5 14:20:57 wily nm-dispatcher: Dispatching action 'down' for usb0
Jun 5 14:20:57 wily systemd[1]: Started Network Manager Script Dispatcher Service.
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): modem state changed, 'disabled' --> 'enabling' (reason: user preference)
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): new Broadband device (driver: 'cdc_acm, cdc_ether' ifindex: 0)
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): exported as /org/freedesktop/NetworkManager/Devices/7
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
Jun 5 14:20:57 wily ModemManager[818]: <info> Modem /org/freedesktop/ModemManager1/Modem/0: state changed (disabled -> enabling)
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): preparing device
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): modem state 'enabling'
Jun 5 14:20:57 wily NetworkManager[836]: <info> (ttyACM0): device state change: unavailable -> disconnected (reason 'none') [20 30 0]
Jun 5 14:20:58 wily ModemManager[818]: <info> Modem /org/freedesktop/ModemManager1/Modem/0: 3GPP Registration state changed (unknown -> registering)
Jun 5 14:20:58 wily ModemManager[818]: <info> Modem /org/freedesktop/ModemManager1/Modem/0: 3GPP Registration state changed (registering -> home)
Jun 5 14:20:58 wily ModemManager[818]: <info> Modem /org/freedesktop/ModemManager1/Modem/0: state changed (enabling -> registered)
Jun 5 14:20:58 wily NetworkManager[836]: <info> (ttyACM0): modem state changed, 'enabling' --> 'registered' (reason: user-requested)
Jun 5 14:20:58 wily NetworkManager[836]: <info> (ttyACM0): modem re-registered; re-checking autoconnect
Jun 5 14:20:59 wily ntpd[3275]: Deleting interface #8 usb0, fe80::8cf4:f3ff:fe50:cfdd#123, interface stats: received=0, sent=0, dropped=0, active_time=29 secs
Jun 5 14:20:59 wily ntpd[3275]: Deleting interface #5 usb0, 192.168.42.109#123, interface stats: received=4, sent=5, dropped=0, active_time=29 secs
Jun 5 14:20:59 wily ntpd[3275]: 91.189.89.199 interface 192.168.42.109 -> (none)
Jun 5 14:20:59 wily ntpd[3275]: 212.43.246.10 interface 192.168.42.109 -> (none)
Jun 5 14:20:59 wily ntpd[3275]: 212.83.179.156 interface 192.168.42.109 -> (none)
Jun 5 14:20:59 wily ntpd[3275]: 91.121.169.20 interface 192.168.42.109 -> (none)
Jun 5 14:20:59 wily ntpd[3275]: 109.69.184.209 interface 192.168.42.109 -> (none)
Jun 5 14:20:59 wily ntpd[3275]: peers refreshed

There are traces throughout the wily's log of upstart errors. Might be systemd migration issue ?