When I upgraded my desktop to Ubuntu Gutsy Gollum (7.10), Pidgin (what was Gaim) started doing odd things. It would connect to Jabber, but then shortly after disconnect. The reason it gave is "XML not well-formed." Messages sent from that account disappeared into the ether, though it could receive messages just fine. There's a bug filed for this on launchpad. The bug was filed in May, five months ago. So much for quality control.
Still, the fault is not entirely with Pidgin or Ubuntu. Even though Adium, at least, had no problem, there was a misconfiguration in my ejabberd server. I was getting this in the logs:
=ERROR REPORT==== 2007-10-14 08:27:42 ===
E(<0.249.0>:acl:178): Wrong ACL expression: [{user,"larry","example.com"},
{user,"bob","example.com"}]
Check your config file and reload it with the override_acls option enabled
I have no idea what the override_acls bit is for, and uncommenting it didn't make any difference. But fixing the broken ACL expression (I guess an array isn't valid here, instead use two ACL expressions), fixed the pidgin problem.