I tried disabling every plugin and it didn't help, I also checked the error logs... there are no errors in there.

The weird thing is, we cloned the website to a different subdomain for testing and it works fine there...

There are no errors in the console.

The only code change we've made to was to the reboot theme, in the file spFunctions.php we changed one value from true to false and when reverting this, it doesn't resolve the issue. The change was:

if (!defined('SP_TOOLTIPS')) define('SP_TOOLTIPS', false);

I've tried all of these options in various setups, none of them change anything in how the forum loads (it loads the same from a visual aspect in every instance)

I've also tried useing the housekeeping cache flushing:

The JS optimizer we use has the forum exempt from optimizing too and even if I disable this optimizer fully it doesn't resolve it:

add_filter('autoptimize_filter_js_noptimize','autooptimize_no_js_optimization',10,0);
function autooptimize_no_js_optimization(){
     if (strpos($_SERVER['REQUEST_URI'],'/forum/')!==false) {
          return true;
     } else {
          return false;
     }
}