Error message from customize button

I am using the following WordPress theme (MH Magazine) on my Website: Furnishfine.com
When I click the “Customize” button I get the following error message:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Web Server at furnishfine.com

Any advice on what to do would be appreciated.