[Dovecot] "storage error during logout"

What is this error message actually trying to tell me?

May 28 13:47:24 yyy dovecot: POP3(aaa@bbb): Disconnected: Storage error during logout. top=0/0, retr=2/73195, del=0/435, size=13509375

This user is accessing his mailbox using both POP3 and IMAP, I don't know if that's relevant. The POP3 check is quite aggressive; once every minute or so.

Dovecot 1.0.13-1~bpo40+1

-- Vegard Svanberg vegard@svanberg.no [*Takapa@IRC (EFnet)]