Mats, a couple of suggestions. Once you have changed the hostname, restart the jabber server by disabling and re-enabling it in the configuration panel.
Second go to console and at root prompt type
[root@sme5 /root]# service jabber status
and it should come back with
jabber is running ... []
you can also try to telnet to the jabber server
[root@sme5 /root]# telnet 192.168.2.1 5222
Trying 192.168.2.1...
Connected to 192.168.2.1.
Escape character is '^]'.
type in any rubbish, enter and you should get the following reply
Invalid XMLConnection closed by foreign host.
Let me know the results
Jon