iChat can't communicate with Jabber account

MBP late 2009, Mac OS X Snow Leopard, 5.0.3

In the past, I was able to connect to:

- Facebook chat

- MSN through Psi

- Google chat

Now all of a sudden, only the Google account works.

Paswords, ports, usernames ... are all ok. Amongst the suggested solutions, I flushed and changed DNS to 8.8.4.4 without luck, reset video ram, cleared caches, reinstalled Microsoft Messenger, disabled the firewall, used another ISP & router, etc. Note that using Adium or Microsoft Messenger does not help either. It is therefore, in my view, a general problem with messaging services (except Google) in OS X.

I found many similar cases on internet, some of which automatically disappeared after a while. I've had this in the past too, but it's suspicious that two such services are broken now (they are not on a windows computer). I found no solution yet after hours of Googling.

Any idea? If I need to find errors for troubleshootig, please instruct the steps, as I use a Mac out of illiteracy.

Thanks !