Ok, we looked at my brother's replay and selected wich cars are coming just after my volontary disconnection, the moment where i can't never rejoin the server.
Those cars are the black RPM Marcos, Lucas Degen's Cupra and the David Garcia's staff car. We joined the STC server today, i took my Saleen and my brother tried each suspect cars. No problem occured.
We tried also to do a server with 24 AI, my brother took the David Garcia's car (the most suspect) and i took again my Saleen, and no problem.
I think it isn't a car related problem, from my point of view it's related to the multislot patch because the problem appear only when we are most of 25 peoples (this what i see on our replay).
Maybe if you can put the same number of car as an STC standard race (with AI) on your server, i can redo one other test to confirm.