Reply To: Theme Customizer Not Working || Showing Page Not Found Error

There is 503 error generated by your server https://gyazo.com/f70dcd91bfd62f72743eff7dd4a7ed13
Try to contact your hosting provider for help on this matter.
500 Internal Error is a generic error and says about some issue on the server and can be caused by different things. Anyway, to understand its nature you need to see your server error logs or enable PHP error display on the website. If you can’t do this, you need to contact your hosting provider for help in this question. Here is an article that may help you understand the error better http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/

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