A customer upgraded from 13.04 to 13.10 and their internal wiki was broken afterwards; for a simple Apache install where the wiki was installed at the web root and all access was via the LAN (thus they were happy with not restricting the access), the fix was to add:
<Location />
Require all granted
</Location>
towards the bottom of:
/etc/apache2/sites-enabled/000-default.conf
…just above the last line, which should be </VirtualHost>.
Restart apache with:
service apache2 restart
After that you should be able to refresh and see your wiki as before.