[Dovecot] utime() failed with index file [...] Operation not permitted

Hello,

I'm running the dovecot backports from jones.dk. Everything is working fine so far, but whenever someone is logging in (IMAP or POP 3) I get the following error message in my mail.log:

[...] pop3(frederik): utime() failed with index file /var/mboxes/frederik/.INBOX/.imap.index: Operation not permitted

[...] imap(jako1): utime() failed with index file /var/mboxes/jako1/.INBOX/.imap.index: Operation not permitted

Rights for the ".imap.index" file are:

-rwxrwxrwx 1 Debian-e mailserv 3792 Sep 21 11:50 .imap.index

Thank you very much for your support.

-Stephan