I made the first fix. I still have an error from times to times (otherwise it works fine), but at least it should no longer be related to VideoMode, like the backtrace showed in one of your previous posts.
That's the first step, now it's going to be more difficult if X is really unable to handle multi-threading properly, as all the windows and contexts in SFML are shared and thus use the same X display :?
I guess you can work around that by using mutexes, so that you can continue to work until I fix all this stuff.