The website's database servers were recently upgraded and reconfigured to a different database format that is radically faster but that does not support text searching. This was a necessary measure to keep everything running smoothly in all areas of MyAnimeList.
As a result, the Forum Search functionality has been temporarily disabled. This includes the Find All links next to your post count in user profiles.
Search should return as soon as a workaround can be setup, so please be patient and do not post new topics about this issue. Sorry for the inconvenience.