gnus hangs while updating nnimap groups buffer

[Martin Monsorno]

Hi *,

when starting Gnus or updating the group buffer with 'g', the
minibuffer says

   nnimap: Checking mailboxes ...

and then nothing more happens.  I can stop it with 'C-g', but the
nnimap groups are displayed this way:

          INBOX                                     * / 0

But I can enter these groups, and read all mails, also new ones. 

I defined an *imap-debug* buffer, but it does not help me any further,
because the output there is produced /after/ hitting C-g.

What's going wrong here?

-- 
Martin