What I’m seeing is: when I try to change error pages from the CPanel I get a 403 forbidden page with “You don’t have permission to access /panel/indexpl.php on this server.”.
I’m using this software: Microsoft Edge Insider (Chromium)
Additional information: It was doing that since more than a month.
If I had to guess, I would say the problem is caused by bad validation rules in the control panel. Can you please try changing the error pages to just http://pocket-inc.ml/error.php, so without the code argument? If that does work, you can also go into the .htaccess file yourself and add extra parameter there.