[Dr.CustUmz]

On 5.2 try downloading your includes/config.php and place the disable plugins code at the very top so the top of your config file should look like

<?php
define('DISABLE_HOOKS', true);

Then upload it back and see if your site loads up

Try yoursite.com/admincp also

If you get in turn your boards off for now and give your members a message

And if that works, a plug-in is causing the issues.

Now if that doesn't work the last thing I can think to do is upgrade to the latest version of vb3 but first have them switch it to php 7.2

I'd be careful with this, already having issues, MAKE A BACKUP, files, db anything you need.