Gnus and nnimap difficulties

[Tassilo Horn]

Hello,

These days I created my first IMAP account and now am fiddeling to get
it working as I want it. Most basic things work. I subscribed the IMAP
folders in the server buffer. I can read my mail and copy/move
mails arround in my (IMAP)-folder.

But one thing is weird: I cannot delete mail (with `B DEL') in IMAP
folders. Gnus always says: 

         "Couldn't delete articles (<article_numbers>)".

I tried it with Gnus-5.10.6 and a current NoGnus with the same result.

Here are my settings:
(setq gnus-secondary-select-methods 
      '((nnml "")
        (nnimap "GMX"
                (nnimap-address "imap.gmx.net")
                (nnimap-stream tls))))

My authorization infos are in ~/.authinfo.

My email account is at GMX.de. Do they have a "bad" IMAP support or is
it my fault?

Regards,
        Tassilo