This question has been answered by ispasov. See the first response.
Is there an .htaccess file in the MODX root? An error there, or a non-modx .htaccess file is a common cause of 500 errors.
If you're using PHP 7+, it has a tendency to throw a 500 error on hitting a PHP syntax error that would pass unnoticed in earlier versions.
Check the MODX error log.
If that doesn't help, sometimes, you can catch the error by watching the network tab in the developer console in Chrome or Firefox (Ctrl-shift-i) and see an error message when you click on a line and look at the response tab. It will be wrapped in some very ugly HTML but the message will be in there somewhere.
I don't think it has anything to do with PHP. The error messages suggest a file permission or ownership issue with the root and connectors directories and/or an incorrect setting in the config files. I think the second one is more likely.
Check the paths in the core/config/config.config.inc.php file, especially the connectors path.
Also, check the core path in the config.core.php files in the MODX root and connectors directories.
Check the site_status System Setting in System (gear icon) -> System Settings.
Also, try renaming your php.ini file. It may have a setting in it that your sever doesn't like.
This line from your error message is very odd (unless it's a typo in your post):
I don't know how the double slash would get there unless it's in the config.inc.php file.