Customize.php no longer viewable in any theme or browser

I am at the end of my patience with this. We have multiple domains, and one in particular, which unfortunately is also our primary one, is having issues with the customize.php.

The “customize,” view when entering “Themes,” is no longer displaying. The layout of the sidebar and main window are visible. The “x” for closing the display and a text box stating, “You are now customizing,” and the name of the site. The “Save and Publish,” button is also live and displaying. That’s where it ends. There are no other options below that and the primary main window is just a blank gray area.

It is only on this site, so clearly it’s something within the Word Press file itself as all the other sites have updated to 4.1 as well.

I have tried disabling all plug ins, and then re-activating one at a time.

I have tried clearing the cache and deleting cookies on all browsers I use. (Chrome and Firefox). I then reset both browsers back to default and scanned them for repair patches as well as made sure both were current. Since it’s only doing it on the one site, I doubt this is an issue, but I was trying to cover my bases.

I have tried downloading the entire Word Press set-up files, (copying WP Content and wp-config.php) deleting the existing and then re-installing all other files. Nothing – still doesn’t work.

Everything else in the editor works, menus, widgets, etc. I can also switch Themes, but cannot customize or change anything about their appearance from within WordPress either. I am running “Pinboard,” as our theme of choice, but regardless of which I choose, they all display the same in the Customize section – blank.

For now, the site is running and works, but does not look like how we would prefer it to and the mobile version is horrible. I would like to find a solution as quickly as possible.

I also searched the WordPress Support for other users posting similar issues and reviewed the suggestions there, but none worked or weren’t relevant to my experience.

Suggestions?