Thank you for your patience and tests. When you was unable to send e-mails through the forum script with PHP 7.4, we can assume, anyone else will not be able to do so. We will have to search for the issue. Please stay tuned for possible questions.
I will... What if my host will drop support for PHP 5...?
And by the way, will it be possible to switch back to use PHP 7.x, using the same database? This is all a mystery for me.