I've implemented a new fix for the thread titles bug.
Hopefully, this should fix it permanently, so it doesn't come back again.
The fix turned out to be very simple: clear the list of filtered words in the forum. For some reason, although there were only 3 filtered words, and they have not changed in years, the system stopped being able to process them. So, I've turned off that filter, and things work again. Sorry for the inconvenience.