It sounds to me like a permissions problem...try running this command:
ls -l /var/log/XFree86*
On the left, there should be something like -rw-r--r-- (that's mine). Towards the middle should be "root root". If there's no w, or there's no root, you need to change your permissions.
If there's no w:
chmod 644 /var/log/XFree86*
If there's no root:
chown root:root /var/log/XFree86*
Hope that helps...