connection to wireless lan gets disconnected

When I connect to wireless lan with knetworkmanager then I get a working connection, but after a few seconds the connection will be disconnected.
Until the connection gets disconnected I have full network access (internal net and internet).

After the second try the connection works without any problems and does not get disconnected.

Here the logs from /var/log/messages (first try):
Nov 25 19:02:55 linux-schleppi kernel: [ 110.032000] ADDRCONF(NETDEV_CHANGE): eth1: link becomes ready
Nov 25 19:03:04 linux-schleppi dhcdbd: dhco_input_option: Value -1 cannot be converted to type L
Nov 25 19:03:04 linux-schleppi dhcdbd: dhco_parse_option_settings: bad option setting: new_dhcp_lease_time = -1
Nov 25 19:03:04 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.host_name
Nov 25 19:03:04 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.domain_name
Nov 25 19:03:04 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.nis_domain
Nov 25 19:03:04 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.nis_servers
Nov 25 19:03:30 linux-schleppi dhcdbd: dhco_input_option: Value -1 cannot be converted to type L
Nov 25 19:03:30 linux-schleppi dhcdbd: dhco_parse_option_settings: bad option setting: old_dhcp_lease_time = -1
-> after the last to messages the connection is no longer available and knetworkmanager shows that there is no connection established.

And here the logs for the second try:
Nov 25 19:08:32 linux-schleppi kernel: [ 446.656000] ADDRCONF(NETDEV_UP): eth1: link is not ready
Nov 25 19:08:33 linux-schleppi kernel: [ 447.896000] ADDRCONF(NETDEV_CHANGE): eth1: link becomes ready
Nov 25 19:08:37 linux-schleppi dhcdbd: dhco_input_option: Value -1 cannot be converted to type L
Nov 25 19:08:37 linux-schleppi dhcdbd: dhco_parse_option_settings: bad option setting: new_dhcp_lease_time = -1
Nov 25 19:08:37 linux-schleppi dhcdbd: dhco_input_option: Value -1 cannot be converted to type L
Nov 25 19:08:37 linux-schleppi dhcdbd: dhco_parse_option_settings: bad option setting: old_dhcp_lease_time = -1
Nov 25 19:08:37 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.host_name
Nov 25 19:08:37 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.domain_name
Nov 25 19:08:37 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.nis_domain
Nov 25 19:08:37 linux-schleppi dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth1 for sub-path eth1.dbus.get.nis_servers

anybody has an idea how to fix or work-around this problem?