Would be great to know how you solved it and what was the reason... if possible, please post here.
Unfortunately, I just could not figure out why it wasn't working. I did a fresh reinstall of a new forum with the most recent version and reinstall the PM add-on. Still whenever I try to delete a message it would return me to a white screen (index.php?).
But it would complete the task when I'd go back to check the message was deleted.
I just could not figure it so, I had to put back up my old minibb forum (miniBB version 2.4.1). And to resolve the http 500 error I was getting after the php version was changed to 7.3 (which was the reason for all of this, my host told me it was an issue with the forum script, and I should upgrade the forum). As a workaround I've put the old forum on it's own domain/directory and have it still running with the old PHP version 5.6.
If you have any idea on how to get the old forum to run on PHP version 7.3 (and higher) or how I can run the PM add-on on the upgraded board, I'd appreciate it. For now I am just making due with what I have. :)