The symlink libglx.so was not there. I added it. But still not result. The strange thing is that I got it working : I ran glxgears at 220FPS ! But then I rebooted, and get the error message I posted again.
I'm currently trying to reinstall everything once again, to see if it works. But up to now, no result. I'm a little bit lost in all symlinks and .so files and all the lib directories !