For the tagcloud.php fatal errors, or a blank screen on search.php, run the following MySQL query:
Code:
UPDATE contenttype
SET cantag = '0'
WHERE class = 'vwPage'
LIMIT 1
After this change you may have to go to AdminCP > Maintenance > Clean System Cache. If you don't have this option, try running VaultWiki's upgrade script, even if you are already on the latest version, since the upgrade script will rebuild the cache too.