[imp] Problem with maintenance using french langage

[Cyril Lamy]

Hi,

I have a mailserver (Postfix-Courierimap-Postfix) running imp 3.1 
downloaded from the main site.

My users made the monthly maintenance for December.
Imp is running in french langage.

Now, my users have the following error :

Warning: imap_utf7_decode: Invalid modified UTF-7 character: `é' 
in /var/www/html/mail/horde/imp/lib/Folder.php on line 164

Warning: imap_utf7_decode: Invalid modified UTF-7 character: `é' 
in /var/www/html/mail/horde/imp/lib/Folder.php 

The name of the folder is Sent-déc-2002

Is this a known problem ?
How can I fix this problem without reinstalling completely imp ?
What files should I modify to correct this ?

Thanks.