An HTTP 500 error will typically involve a PHP error that gets written to a file called “error_log” within the folder of the install that is throwing the error. In this case it appears the error is:
[10-Feb-2018 02:37:04 UTC] PHP Fatal error: Cannot use 'Null' as class name as it is reserved in /home/marksmat/public_html/forum/plugins/oembed/library/vendors/fg/essence/lib/Essence/Log/Logger/Null.php on line 20
It looks like the oembed plugin is not functioning. I do know we recently have started rolling out PHP 7 on our servers so as a test I switched your account back to PHP 5.6 and that seems to have worked. If there are updates to Vanilla Forums or that specific plugin it would be a good idea to run them since as time goes on PHP 5.6 is not going to be supported for long (it is end of life and has been for awhile, this year it will stop receiving security updates and may lose support within cPanel at some point in the future).