[Dovecot] 1.2.7: recs[i]->uid < rec-> uid

I'm getting this Panic with some users on dovecot-1.2.7:

Panic: file maildir-uidlist.c: line 1242
(maildir_uidlist_records_drop_expunges): assertion failed: (recs[i]-

uid < rec-> uid)

There's another dovecot-1.2.3 running on identical hardware accessing
the same NFS mail storage without problems.