Had problems with this error today and spend ages trying to find the answer. I then realized that the upgrade to MySQL recently (I think 5.1) had probably been the cause. Updating through WHM and Cpanel just sometimes is not very thorough.
This command solved it for me in the end
# /scripts/mysqlup --force --verbose
# /scripts/restartsrv_tailwatchd
Thanks to this page for the tip.