Thinking out loud.
With everything said.
You should understand that your version does not use php nor mysql..
It only uses perl. To a point unless you use the php accelerator.
So technically it should still run with the server update you had..
1:) perl is installed to support cgi files( This is so old I don't recall what versions are supported.)
2:) path to perl has not changed-(This can be corrected by editing a few classic files if it has changed:ultimatebb.cgi, cp.cgi, and ubb_upgrade.cgi.)
3:)Your version should have a accelerator php file that does use php., If you turned it on then it may attempt to run php and may crash if the new php version is not compatible.
This can be turned off via the ubb control panel. Or ftp to the site and edit the var files.( If you can access the control panel this probably is the problem, Since only the main forum will use it if the path to Perl has not changed)
Looking at your post the control panel should be at /cgi-bin/cp.cgi
But you need admin access to login.
The setting is at:
Primary Settings. On the General tab
Also if path to Perl has changed you will need to edit at least ultimatebb.cgi, cp.cgi, and ubb_upgrade.cgi at the top of the page.
Last edited by Ruben; 10/21/2018 3:17 PM. Reason: Added comment.