Internal server errors are not specific to WordPress, they can also happen as a result of anything else running on your server as well.
An internal server error is very generic, therefore it does not tell the developer anything. It does not specify the cause of the error or where the problem is.
Increase the PHP Memory Limit
Sometimes this error can show up if you are exhausting your PHP memory limit. Ask your hosting provider about your server settings and limitations. There may be a very small limit set there and any script that needs to process bigger jobs will be stopped.
Check Server Permissions
On some servers based on firewall conditions, or other restrictions, certain files (from subfolders, or with specific names) are restricted from being accessed via browser. Ask your hosting provider what error was stored in the server logs behind the generic “500 Server Error”.