I searched this Message Board and found some solutions to this problem. The main solution is to choose another server because the server you chose first was unavailable. My problem is that I get this message at all my tries to connect, no matter what server... what could be the explanation of this problem and what could be the solution?

1. enable ident (alt + O > connect > identd)
2. try another network just to make sure it's not network-related (your isp may be banned)

Nope, it is with every network....