I just realized I posted this in the wrong thread; my problem is on b77, not HoF.
And even though I first noticed the issue with Tapatalk, it isn't actually related to that. The problem is that my account (and just mine?) doesn't have permissions to use search anymore (Tapatalk apparently makes use of search to show the "unread posts" list, etc.). Clicking on "Search" at the top of the screen just brings up a page that says "sorry but you are not permitted to use the search system." Logging out and back in didn't help; neither did clicking the "delete all board cookies" button. It was all working fine a few days ago.