[Dovecot] Mailbox autocreate feature cannot create folders

[interfaSys sàrl]

Hello,

The "new" way of creating default mailboxes doesn't work for us. There
seems to be a permission problem.

We get this in the logs:
imap(testuser at domain.com): Error: nfs_flush_file_handle_cache_dir:
rmdir(/var/mail) failed: Device busy

That error message has nothing to do with NFS (not used) or /var/mail
and appears every time an app is requesting a list of folders via IMAP
and only if we enable the autocreate feature.

We're using mdbox, emails are delivered via LDA and the folder
permissions look like this:
# la /home/user/imap/domain.com/testuser/mdbox/mailboxes/
total 5
drwx------  3 user  mail     3B Jan  5 19:33 INBOX
drwx------  3 user  mail     3B Jan  5 19:33 .
drwx------  4 user  mail     8B Jan  7 02:43 ..

The mailbox definitions used come straight from the wiki:
http://wiki2.dovecot.org/MailboxSettings

I've enabled debugging, but there is no mention of the mailboxes to create:
Debug: dict quota...
Debug: Namespace : type=private, prefix=, sep=/, inbox=yes, hidden=no,
list=yes, subscriptions=yes location=mdbox:~/mdbox
Debug: fs:...
Debug: acl:...
...

Here is what works:
- Autocreate plugin
- Creating folders via various clients

So, it seems Dovecot can create folders, just not using that feature.
If that's not a bug, would there be a way to make it work without
altering permissions?

Cheers,

Olivier