Wrong number of new mails in nnimap group

[Hermann]

Hi,
I switched the mail backend to nnimap, and since yesterday I observe the
following phenomenon, that wasn't there before:
When new mails arrive, Gnus shows wrong numbers. When one mail arrives,
I see between 10 and 20 new messages.
When more new mails are in my inbox, it happens that I get several
thousands announced. In fact there are 3 or 4.
I also use the same Imap account with my notebook with Windows XP and
Thunderbird. (It doesn't happen there)
What may cause this? It's no show stopper, but annoying.
Hermann