Umm..sounds odd, but make sure the file /var/log/XFree86.0.log
really exists and is writable. Or use some other logfile for debugging purposes. Did you try to run startx
(for example) as root, or as regular user? It makes difference, and if you have not tried the other way, try.
Check the file's existence and permissions:
ls -l /var/log/XFree86.0.log
if you get the file information, it exists -- check out it can be written to by the user who runs X. If it does not exist, create it (either as the user who runs X, or then alter permissions after creation; wrong permissions cause trouble):