Help debugging a search problem

I'm having a problem with the search, when I search for a work I get "You must include at least one positive keyword with 3 characters or more."

the search it's indexed, cron it's running, I've tried rebuilding the index and nothing it's working. I don't have strange messages on watchdog.

I'm using 6.14 and the search_restrict module and I recently installed memcached on the server. The search was working and I have no idea when exactly it stopped working.

Thanks in advance