Server Error when accessing Customize in the WP dashboard

Thank you Tom. Yes, I believe you are correct. IT is now involved. Updated PHP, checked the server environment, reinstalled Wordpress, killed all plugins, and it still does not allow access to "customize" in the dashboard menu.

I received this message from IT: The good news is there is nothing wrong with our environment--Wordpress 4.8.1 is fine, IIS is fine, PHP is fine, MySQL is fine, and our configurations are fine. The bad news is that means there is something in our site's database which is old, crusty, and causing the customization page to die.

It's not GP, but any thoughts you have, we're all ears.

Hmm, well an internal server error should print the exact error in the error_log file. Is there nothing in there?

Fixed. A widget with some corrupt data was the cause. Had nothing to do with GP. Thanks for the quick response. High marks for your support.

Good news! Thanks for reporting back :)