Someone in the past has suggested a *nix cron job that goes wrong: I'm now wondering if that is tied up to my usual grumble about the cache directory not being writable.
phpBB is well beyond my pay grade, sadly. I know from searches that there are issues with phpBB 3 (and one of them is indeed "cache NOT writable).
However, nonetheless I think it's great that Merk has given us a free-to-use playground for so long - I've just had a mod on another forum threaten me with closing my account after he challenged me over using an adblocker/tracker blocker and not subscribing. He got handed his arse on a plate by a couple of the regulars.