[Dovecot] irregular problem on outlook + imap

Hi list, my customer say me that "sometime" he has a problem with imap server when he try to move a mail from a folder to another. The error are: """ NO [TRYCREATE] Mailbox doesn't exist: folder.with.dots and spaces and numbers"""

Like now I had connected to the server for create a "tcpdump | strings" session and the results are:

vj8q OK Idle completed. can9 LIST "" "INBOX" dtZ@

• LIST (\HasChildren) "." "INBOX" can9 OK List completed. zp2p LSUB "" "*"
• LSUB () "." "a_folder" <-cut folder list->
• LSUB () "." " folder.with.dots and spaces and numbers" <-cut about 600 folders->

and at the end:

zp2p OK Lsub completed. 3j1@ znem IDLE 2tw@

• idling (jI@ DONE Atx@ znem OK Idle completed. lylu APPEND "folder.with.dots and spaces and numbers" (\Seen) "30-Nov-2009 10:34:00 +0100" {280844} lylu NO [TRYCREATE] Mailbox doesn't exist: folder.with.dots and spaces and numbers xbci IDLE

On the maildir home (maildir:~/mail/), it's present the folder.with.dots and spaces and numbers and the same into the "subscriptions" file

My env: debian lenny with last dovecot (1.0.15-2.3) On the log, there is nothing special.

What can be?

Thanks, Michele