Ok, in a user's profile, clicking 'Postings' and then clicking 'Find all threads by USER' it shows threads for everyone instead of that user. I disabled all addons and the problem persisted. How can I debug this?
Well the standard search is only "supported" with MyISAM -- there are obviously some differences coming up with InnoDB's version that make it not function as a drop-in replacement. I could only recommend leaving it as MyISAM.