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500 Internal Error 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/
Yes, we see this 500 error but it seems to be caused by some PHP Fatal Error. Contact your hosting providers and ask them to provide you with error logs. Or enable PHP reporting on your website.
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