hmm,
Can you post your most recent stacktrace?
/opt/zimbra/tomcat/logs/(stacktrace with the newest datestamp)
From here it looks like an LDAP problem:
Code:
Caused by: javax.naming.CommunicationException: zimbra.nu2m.de:389 [Root exception is java.net.ConnectException: Connection refused]
I think your LDAP password may have gotten out of sync, perhaps. When it crashes, try telneting to the machine on port 389, and see what happens.
john