A "500 Error", also called an "Internal Server Error", is an error generated by the server, and it masks the actual error that caused the 500 Error.
It's like the "Check Engine" light on a car. It says something is wrong, but not what or where.
You'll need to contact your Host, and ask them to search the Apache Error Log (not the PHP error log) for the 500 Error entries recorded for your IP Address. Those entries will show the actual underlying error that triggered the 500 Error.
But if the errors aren't clear as to cause and resolution, post a followup here.