strange error message with imap

[Armin Lambacher <lambache@biochem.mpg.de>]

Hello,

I'm new to IMAP, so maybe this is a stupid question, however, I could
not find anything related to it in the info files.

Whenever I leave one of my imap groups I get an error message similar
to the one following:

----------------------begin messages--------------------------------
Retrieving newsgroup: nnimap+mail:general...
nnimap: Updating info for nnimap+mail:general...done
Fetching headers for nnimap+mail:general...done
Generating summary...done
Expiring articles...
error in process filter: imap-parse-response: Internal error, tag 226 status BAD code nil text Bogus criteria list in UID SEARCH
error in process filter: Internal error, tag 226 status BAD code nil text Bogus criteria list in UID SEARCH
Expiring articles...done
----------------------end messages----------------------------------

The number is different each time, the text is the same.

What happens?

Here is the content of my .gnus file, I'm not aware of having
customized anything via customize...

(for sake of space I have omitted some lines of my nnimap split rules which are essentially of
the same kind as the ones presented, just different names...)

----.gnus-----------------------------------------------------------------
(setq gnus-select-method '(nntp "news.biochem.mpg.de"))
(setq gnus-secondary-select-methods '(( nnimap         "mail"
                                        (nnimap-address "mailin.biochem.mpg.de")
                                        (nnimap-nov-is-evil t)
)))
(setq nnml-directory "~/gnus-Mail")
(setq gnus-visible-headers '("^From:" "^Subject:" "^Date:" "^X-Sender:" "^To:" "^Cc:"))
(setq gnus-message-archive-group
   '((if (message-news-p)
        "misc-news"
      "misc-mail")))
(setq gnus-show-mime t)
(defun my-set-thread-sort ()
  (setq gnus-thread-sort-functions 'gnus-thread-sort-by-number))
(add-hook 'gnus-summary-generate-hook 'my-set-thread-sort)
(setq gnus-posting-styles
           '((".*"
              (address "lambache@biochem.mpg.de"))))

;
; IMAP Mail splitten
;
(setq nnimap-split-inbox
    '("INBOX")
)
(setq nnimap-split-crosspost nil)
(setq nnimap-split-rule
  '(
    ("xfree86.newbie" "To:.*newbie@XFree86.Org")
    ("gsl"            "To:*.gsl-discuss")
    ("dri-users"      "To:.*dri-users@lists.sourceforge.net")
    ("general"    "")
   )
)
---end of .gnus--------------------------------------------------------------

Can anybody help me?

Best regards 
Armin Lambacher