Problem resolved:Admittedly the Jabber clients for MacOS are sub-optimal. Hopefully that situation will be remedied in time for San Francisco.
(a) JabberFox works. I have no clue why it didn't the 50 times I tried earlier yesterday. Now it does.
(b) TVJab is unable to connect to a chat room at a conference server which is not the same/known by the jabber server you connect to. I don't really know if this is a server configuration, or bug/lack of feature in the client.
So, my recommendation: If you use MacOSX, JabberFox is what you should go with.
THANKS to everyone which have sent me private email!
paf