ok, I think I have identified the problematic package : libx11-6 2:1.1.1-1 (experimental) and libx11-xcb1(experimental) are on my system. there are the only 2 X related packages that are different from my system and another (sid too) where it works. I already had some problems and incompabilities with libx11-xcb1but though it was migrated to sid with xserver-xorg and I forget it when I firstly see the bug.