Big extrange problem with cyrus imap

[Eduard Pertíñez i Juncosa]

Hy I have a debian box where I recompiled and Installed Cyrus-imap 2.1.18 
against Berkeley DB3.

After lots of tweaking and trying I made the PostFix-CyrusImap bundle work.

But now I have a blocking problem.

I cannot connect to the IMAP folders more than once at the same time. The very 
first of the users gets connected but the rest start the connection and it 
stops working, like if something was blocking it.

It happens to all users and programs: kmail, imtest, webmin,...

The very first connects. The rest stop waiting for a reply.

I've been unable to find a logical explanation, because nothing is showed at 
the logs (that I've seen).

I've been googling around but nothing found.

Does anyone of you found a similar problem? Where could I start looking for?

Thanks in advance,

Eduard