[imp] UNable to auth IDN in IMP - log data inline

[info at interpost.biz]

I have this entry for IDN IMP failed auth attempts in the syslog:

Aug 10 00:58:15 server1 HORDE: [imp] [login] Authentication failed.  
[pid 11357 on line 732 of "/var/www/horde/imp/lib/Imap.php"]
Aug 10 00:58:15 server1 dovecot: imap-login: Disconnected (auth  
failed, 2 attempts in 8 secs): user=<info at boîteauxlettres.net>,  
method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, TLS,  
session=<WEzESD8A0gB/AAAB>

The user= should read boîteauxlettres.net .

I also note that no log file for imp is being created in /tmp as the  
backend.php file indicates it would, and I am checking my permissions  
to see if I can get the imp auth log file to be created where  
indicated in the backend.php file.

I am using sql authentication - this is latest upgraded debian wheezy  
with php 5.5.latest stable

Horde is running on top of postfix/dovecot, and there are no problems  
with non-IDN domains.

What do you recommend?

Would changing the collation of the horde user table to something  
other than the current UTF-8_general_ci, and inputting the same  
collation in the horde conf.php file, work?

Or would I have to change the collation of the entire horde db?

Would enabling mbstring conversion and applying it only to horde or  
mail possibly work? (this is a last resort because I have had big  
problems enabling this before).

I need to get IDN's working on horder latest, and am willing to use  
this as a test server/setup if you have ideas on how best to get this  
to work with IDN's.

Thank you for any ideas of help / recommendations.