It could be useful for some old installations:
error.log is full of:
PHP Warning: preg_match(): Compilation failed: group name must start with a non-digit at offset 8 in /var/www/html/wiki/includes/MagicWord.php
All pages are blank, but you can edit them with normal content.
To fix, you need to edit /includes/MagicWord.php like:
Its better to upgrade BTW.