You have the file in your forums/includes/ folder, it is looking for your forum config under forums/includes/forum/includes/config.inc.php
Move the file to your web root, so it is accessible as
http://www.byzcath.org/ubbt_test.php and set $config["path"] inside the file to read "forums" (by default it reads "forum"); it should now process through your UBB.threads configuration.
Yes, some of the possible problems are worrisome... They equate to:
mysql_connect - doesn't matter, your system has mysqli_connect installed since it is PHP7, so it'll use that instead.
file_get_contents - Needs to have allow_url_fopen enabled so that your forum can communicate with
www.stopforumspam.com for spam prevention.
max_input_vars - Needs to be raised to 3000, by default it is 1000, which means you can send config updates fore up to ~30 forums, any more than that and you could end up with your configuration file wiped (which could be the problem here if your config file is empty and you've submitted items from the control panel after the config change).
memory_limit - Your PHP Memory Limit is extremely low, todays standards generally see this value at 128MB for PHP7+.
