This has been an issue particularly with the stats table. 504 is gateway timeout. MySQL takes a long time to optimise stats, and the server gives up waiting.

The task *does* continue, but only for the table which was being optimised at the time of the timeout (later tables don't get optimised).

I recommend using MySQL CLI directly to run these operations instead.

In the future I hope to have actual INNODB support, but that really depends on my availability to implement it.