Hi Didin,
Thank you for your purchase.

Please provide me a temporary WP-admin login for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. You can write credentials here just check Private Reply so nobody can see them except us.

Best regards.

HI Didin.

OK I can see the 502 Bad Gateway error ( attachment) 

This is actually the "502 Bad Gateway error", and it means that one server on the internet received an invalid response from another server. 

502 Bad Gateway errors are completely independent of your particular setup, meaning that you could see one in any browser, on any operating system, and on any device. Bad Gateway errors are often caused by issues between online servers that you have no control over. However, sometimes, there is no real issue but your browser thinks there is one thanks to a problem with your browser, an issue with your home networking equipment, or some other in-your-control reason.

You can read much more about this error on this thread. The first thing to try is to delete your browser's cache and try again, but I believe contacting your hosting provider would be the best and fastest way to go about this.

Basically it is a communication issue, and has nothing to do with your set-up, WP or plugins installed.

Do you have any errors in the server log?

Best regards.