[imp] Weird session openning under horde 2

[LaurentG]

Hello list,

Installing Horde2 & IMP3 on Apache(SSL) 1.3.3 and against CyrusIMAP 
2.2.8, a strange behaviour suddenly occurs :

When loggin in, the right user goes back to the login page.
If he mistakes and then tries again with the right login/password he 
goes to his inbox.

No errors in logs, except for the intentionnal wrong login.
Has anyone got a clue or a direction to investigate?

Thanks for your ideas.
Laurent