lockups since version 2:4.7.6+dfsg~ubuntu-0ubuntu2.9

Since version 2:4.7.6+dfsg~ubuntu-0ubuntu2.9 / Ubuntu 18.04.1 LTS, smbd locks up sometimes. We are unsure why. No messages in the log.smbd.

Symptoms: clients (a few, about 5 to 10, OSX and Linux machines) sometimes just cannot connect to Samba anymore. They call us, "the server doesn't work".

Log messages: none that we are alarmed by (smb.conf log level = 1)

Then, trying to restart samba (systemctl restart smbd.service) says:

Apr 12 14:23:37 kantoor systemd[1]: Stopping Samba SMB Daemon...
[...]
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: State 'stop-final-sigterm' timed out. Killing.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29016 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29017 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29034 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29035 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29046 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29047 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29048 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29049 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29067 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29068 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29077 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29078 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29487 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29749 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29784 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Killing process 29793 (smbd) with signal SIGKILL.
Apr 12 14:25:07 kantoor systemd[1]: smbd.service: Failed with result 'timeout'.
Apr 12 14:25:07 kantoor systemd[1]: Stopped Samba SMB Daemon.
Apr 12 14:25:07 kantoor systemd[1]: Starting Samba SMB Daemon...

While a regular restart, when there are no complaints about "the server isn't working", just goes in sub-second time, like this morning:
Apr 13 10:09:17 kantoor systemd[1]: Stopping Samba SMB Daemon...
Apr 13 10:09:17 kantoor systemd[1]: Stopped Samba SMB Daemon.
Apr 13 10:09:17 kantoor systemd[1]: Starting Samba SMB Daemon...
Apr 13 10:09:17 kantoor systemd[1]: Started Samba SMB Daemon.
Apr 13 10:09:17 kantoor smbd[14174]: [2019/04/13 10:09:17.107277, 0] ../lib/util/become_daemon.c:124(daemon_ready)
Apr 13 10:09:17 kantoor smbd[14174]: STATUS=daemon 'smbd' finished starting up and ready to serve connections

Would it help to try to attach "strace -p ... -p ..." for the smbd processes next time the lockup happens? Or do we need another log level?

The smb.conf is fairly straightforward (maybe a bit oldfashioned):
[global]
        log level = 1
        workgroup = hssa
        passdb backend = ldapsam:ldap://127.0.0.1/
        ldap admin dn = cn=admin,dc=kantoor,dc=hssa,dc=nl
        ldap ssl = off
        ldap suffix = dc=kantoor,dc=hssa,dc=nl
        ldap user suffix = ou=Users
        ldap group suffix = ou=Groups
        ldap machine suffix = ou=Computers
        unix extensions = yes
        delete readonly = yes
        ea support = yes
        ldap password sync = yes
        interfaces = 127.0.0.0/8 ens3
        bind interfaces only = true
        load printers = no
        printing = bsd
        printcap name = /dev/null
        disable spoolss = Yes
        disable netbios = yes
        smb ports = 445
        dns proxy = no
        vfs objects = fruit streams_xattr
        security = user

[homes]
        comment = Home Directories
        writable = yes

[hssa]
        force group = users
        force directory mode = 2770
        force create mode = 0660
        directory mask = 2770
        create mode = 0660
        comment = Algemeen
        writable = yes
        path = /home/hssa
        mangled names = no
        mangling char = _
        valid users = @users

[hssa-intern]
        force group = hssaintern
        force directory mode = 2770
        force create mode = 0660
        directory mask = 2770
        create mode = 0660
        comment = HSSA-intern
        writable = yes
        path = /home/hssa intern
        mangled names = yes
        mangling char = _
        valid users = @users
---
ApportVersion: 2.20.9-0ubuntu7.1
Architecture: amd64
DistroRelease: Ubuntu 18.04
NmbdLog:

OtherFailedConnect: Yes
Package: samba 2:4.7.6+dfsg~ubuntu-0ubuntu2.9
PackageArchitecture: amd64
ProcVersionSignature: Ubuntu 4.15.0-36.39-generic 4.15.18
SambaServerRegression: Yes
SmbConfIncluded: No
Tags: bionic
TestparmExitCode: 0
Uname: Linux 4.15.0-36-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

_MarkForUpload: True