Some users can\'t see the others

[PtitLu]

PFC Beta 11.
Example : Users A & B are logged and can chat. Users C & D are logged on the SAME server and can chat.
But A & B can't see C & D, and C & D can't see A & B. It seems that there are two sessions :/

Quite strange and hard to understand and resolve for me.
Thanks for your help.