[Dovecot] Messages not deleted from mbox

[venom00]

Hello, I've two accounts on my mail server, and from my client
(Evolution) I've set up two configurations. If I dowload mail one
account at a time everything works fine, but if I download both of them
at the same time I'm not able to delete downloaded messages, so I
download them twice, three times and counting to infinity.

On th client side everything looks good:

-----------------------------------------------------
LIST
+OK 1 messages:
1 485
.
UIDL
+OK
1 000000704e57eebd
.
RETR 1
+OK 485 octets
Message here...

.
DELE 1
+OK Marked to be deleted.
QUIT
+OK Logging out, messages deleted.
-----------------------------------------------------

On the server side I've enabled debug logging but nothing relevant comes
up, except sometimes an error:

Panic: Message unexpectedly expunged from index
Full log: http://pastebin.com/raw.php?i=bBizx9tm

But most of the times there are no errors and simply the message is not
deleted.
I'm using mbox format in /var/spool/mail/%u. The dovecot user has
read-write access to the folder (even because, as I said, if download an
account at once everything works fine).

I think it's about locking, I've tried several lock methods but always
the same result and I can't see dead locks from lsof, fuser or ls (for
dotlocks).

Thanks in advance, 
venom00