ewgjt whatever I do last doesn't take effect unless I wait a while before clicking Post reply
Clutching at straws here, but is it possible the system is only sending out the draft message, and if you wait a while before clicking reply it's the new draft that's saved? Maybe some file didn't overwrite properly in the upgrade process? If so, it may be worth trying to upload the files again, ensuring everything is properly overwritten. You could also attempt just deleting all of the files (except for the ost-config.php file) then reuploading everything from scratch, but make sure you have a reliable backup before this.
While it shouldn't be necessary, I usually restart Apache after an upgrade, or the whole server to really make sure things are reloaded properly.
I've never seen this behaviour before either, so unfortunately it's just 'clutching at straws' suggestions I can offer 🙁