Amerikaner08, I've seen this happen before. Yahoo! Messenger hangs when making the HTTP connection to the room list (insider) server. Unfortunately there's nothing that can be done about it but to wait it out.
The last version of Yahoo! Messenger that doesn't have the
ignore bug is version 7.02 which can be found here ->
Download Yahoo! Messenger 7.0.2.120 - FileHippo.com