I did a quick search, and found this on another phpbb forum that was having the same issue…
> the search table was corrupted. they have been fixed. if you still get the same error message, please let us know asap.
http://www.getphpbb.com/support/viewtopic.php?p=16826&sid=4828f8d397691893f647f384b2d325c9
Mal