If you've been using fglrx before, it might be the reason. Try: dpkg -l '*fglrx*' and locate fglrx to see if there is still some proprietary bits around causing problems.
If you've been using fglrx before, it might be the reason. Try:
dpkg -l '*fglrx*'
and
locate fglrx
to see if there is still some proprietary bits around causing problems.