This question was answered by @mikesale:

It is the very definition of a server side error.

You need to review your server logs as you have configured your nginx or apache web server to get the reason for the error.

If you’re using Ubuntu/Apache2 they’re typically found in /var/log/httpd/apache2/ with a fill name of error.log

You may also need to look at your php logs, mine running fpm are found in /var/log/

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