Was running Ubuntu 8.04 LTS 64-bit w/ ZCS 6.0.7. Upgraded to Ubuntu 10.4 LTS 64-bit and ZCS 6.0.8 and have been having strange stops.
I see a variety of error messages in /opt/zimbra/logs/mailbox.log, the common factor I can see is the string:
com.zimbra.common.service.ServiceException: system failure: ZimbraLdapContext
A 'zmcontrol status' shows all services (antivirus, ldap, logger, mailbox, mta, snmp, spell, stats) listed as 'running'. Users experience the problem as rather generic and un-helpful 'Network Error' listing in ZWC.
A restart ('zmcontrol stop', followed by 'zmcontrol start') returns full function.