[Dovecot] Dovecot-1.0-test32 fails with assertion

[dovecot]

Hello,

 dovecot-1.0-test32 continuously fails on one of our customers mboxes
with the following error:

Sep 11 16:27:15 web dovecot: POP3(xxxxxxxxxxxxx.de1): file
mbox-sync-rewrite.c: line 360 (mbox_sync_read_and_move): assertion
failed: (need_space == -mails[idx].space)

 About 200 other mboxes are working perfectly on the same server. Where
should I start looking for the cause of this?

Regards,
Martin