https://bugs.freedesktop.org/show_bug.cgi?id=62578
--- Comment #6 from Marek Olšák maraeo@gmail.com --- The issue seems to be that kwin or DDX allocates a texture of the size 2*1600x1200 and your hardware only supports 2560x2560 for rendering and 2048x2048 for texturing. Unfortunately I'm not an expert on X server internals.