https://bugs.freedesktop.org/show_bug.cgi?id=72159

--- Comment #7 from Michel Dänzer michel@daenzer.net --- (In reply to comment #6)

I redirected the output of startx to a log file, but there's nothing interesting in there AFAICS.

I don't think the stderr output of startx captures the stderr output of the X server. Please manually start just the X server itself and redirect its stderr output instead.