Thank you for posting this solution!

I was having an issue after migrating a database to a new server. After migration the FT indexes worked fine. However, they would break anytime there was an attempt to repopulate the FT indexes. The repopulate attempt would repopulate the index with 0 records. This was apparently due to the ft_timeout being set to 0 on the new server. No clue how or when it got set to 0 but it was.

Updated it to 60000 and everything resumed normal operation.