[Dovecot] irregular problem on outlook + imap

[Michele Petrazzo - Unipex]

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