Hi newox,
Are you still having this issue?
If yes, is the Theme page the only page that comes up blank?
If this issue persists I would try reuploading the theme files (you may as well us it as an opportunity to upgrade to the latest version); and then I would make sure that the permissions are all correct and that the owner of /include/staff/theme.inc.php is the same as the owner of the other files in the /include/staff/ directory.
Let me know!