I also tried the Pro version Helper Plugin 1.5.5. It did not help, same fatal 500 error. I just tried it now. The SNAP Plugin works fine, it is the Pro Version Helper Plugin, which is causing the error. I get these errors:
WP Debug Log: [14-Apr-2018 03:00:22 UTC] PHP Fatal error: Can’t use method return value in write context in /wp-content/plugins/nxs-snap-pro-upgrade/nxs-snap-pro-upgrade.php(40) : eval()’d code on line 60
