500 Internal server error in WordPress can be caused by many things. If you’re experiencing one, there’s a high chance one (or more) of the following elements is causing the issue:
- Browser Cache.
- Incorrect database login credentials.
- Corrupted database.
- Corrupted files in your WordPress installation.
- Issues with your database server.
- Corrupted WordPress core files.
- Corrupted .htaccess file and PHP memory limit.
- Issues with third-party plugins and themes.
- PHP timing out or fatal PHP errors with third-party plugins.
- Wrong file and folder permissions.
- Exhausted PHP memory limit on your server
- Corrupted or broken .htaccess file.
- Errors in CGI and Perl script.
Please refer to below link for more details