[horde] horde3 loop on login.php (imap auth)

[sCALP]

Hi everybody, I'm new on this list, I hope not to bother you, but I 
don't know where to find help, google and the archive didn't help.

I can't manage to configure Horde-3 to auth me using IMAP on my 
courier-imap server.
I don't have Imp-h3 installed yet, I just want to make my Horde log-in 
using IMAP.

everytime I call login.php, I enter my credentials, and it loops back to 
login.php.... ever and ever.

I've got a "success" in the horde log file :
Dec 26 18:47:59 HORDE [info] [horde] Login success for xxx at tibone.com 
[xxx] to Horde [on line 76 of "xxx/login.php"]

and even my imap log file says everything is clean :
Dec 26 18:47:59 xxx imapd: Connection, ip=[::ffff:127.0.0.1]
Dec 26 18:47:59 xxx imapd: LOGIN, user=xxx at tibone.com, 
ip=[::ffff:127.0.0.1], protocol=IMAP
Dec 26 18:47:59 xxx imapd: LOGOUT, user=xxx at tibone.com, 
ip=[::ffff:127.0.0.1], headers=0, body=0

When I configure horde to automaticaly log me with my login/password, it 
works well, and I can even go directly to Imp (if it's installed)
But I want me and my friends to log with their IMAP accounts (login @ 
domain . tld + password), so I need Horde to be able to authenticate 
with IMAP.

I tried to configure Horde to authenticate itself using IMP (which use 
IMAP too) but the result was the same, a loop on login.php

Am I a newbee who can't follow the installations rules or is there a 
trick I didn't know ? (I swear I won't be offensed with a "you're a 
newbee" answer...)

thanks
sCALP