Not all, but several forums, when you click on the main link in the block, give the following error:

Exception - Argument 12 passed to mod_forum\local\entities\post::__construct() must be of the type boolean, null given, called in [dirroot]/mod/forum/classes/local/factories/entity.php on line 157

I went up to php7.3, but also tried it with php7.2 in case that was the problem. Any thoughts?

Alan