After the great help of Sylvie (many thanks for your time) we found the problem.
On the same domain i use Tikiwiki and a external forum (discus). They both use the same cookie as there are on teh same domain.
As my forum doesn't seams to have any problem with that, Tikiwiki read it all then seams to be stuck somewhere with the datas from Discus.
The problem found doesn't mean it is solved. I guesse something should be done to avoid tikiwiki to read anything else than his own data.
Meanwhile i'll try to set a sub-domain for my forum forcing the use of two cookie, one for tikiwiki and one for discus.
I'll let you know about it !