I was incorrect when I said "I'm manually upgrading my Invision Community forum from 3.x to 4.6.10". I'm actually on 4.3.6 and trying to upgrade it to 4.6.10.
Tonight I restored my 4.3.6 forum from backup and have it running in its old location on my apache2 server (/var/www/html/forums), which is setup for PHP5.3 on that apache2 <directory> using php-fpm. Of course, I can't upgrade to 4.6.10 in place there, because 4.6.10 wants PHP 7.0 or later. So, I went ahead and created a new sub domain on my server that is using PHP7.4 just fine at /var/www/forums/ apache2 <directory>. But, if I move my forum to that new subdomain, I get the 500 error I mentioned earlier.
I'm using the local file system for data storage. When visiting the "Data Storage" tab on my 4.3.6 system, it lists the location of /var/www/html/forums/datastorage. If I move the forum to /var/www/forums do I need to redirect that location by hand somehow? I grepped through my forum directory for that path but found it nowhere. Is this the cause of the 500 error? Looking at my php error_log gives no indication on what's going on.
It is my hope to move my 4.3.6 forum from the PHP5.3 directory to the PHP7.4 directory and then hit the upgrade button on it.