There have been a few instances of this error since the upgrade/rebuild. When using SEARCH it happens, also inline moderation. I'll look into it soon, in the meantime if you get it please post the URL etc into this thread.
You will also get such error when using the voting on the main page. (same on EF) Your submission could not be processed because a security token was missing or mismatched. If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.
Thanks itto :) Fixed the search - now I just need to track down the other ones.... update: vote on home page fixed now too :)