Why won't some internal jabber accounts go online since using 10.8 and OS X server's jabber service?

Since we run Apple's "Messages" instead of iChat on all Macs, some Macs won't go online anymore anc connect to the internal iChat/Messages-server on our OS X server 10.6.8 or server 10.8.2 (both tested, no change). We can clean all user caches and restart the clients affected, but it won't change a thing after the next restart. We are really desperate, because iChat/Messages is our communication backbone.