Thunderbird 3.0: "Check mail" doesn't work

Hi there!
I already had problems with the "Check mail" button under Thunderbird 2.x (solved it by using the "Get all mails" plugin). Now after upgrading to Thunderbird 3.0 I have the same problem again. Furthermore the "Get all mail" plugin is not working anymore now, too.
This means: When I try to check for mail via menu, tool bar button or "Get all mail" button just nothing happens at all. Sending mail is working fine.

Any help would be highly appreciated! Thanks!