Customize Publish Button

When I save my changes in the customize theme using the Publish button I get an error message that says: Looks like something’s gone wrong. Wait a couple seconds, and then try again.

AND subsequent saves it says: The previous set of changes has already been published. Please try saving your current set of changes again.

I’ve turned off all plugins and still get this problem. I had another developer look at this and we can’t solve the issue.

The changes are saving, but why are we getting this error message? It’s annoying because sometimes you’re not sure because when you click to go back to the WP admin the browser sends a message: The changes you made will be lost if you navigate away from this page.

The theme is current and the WP install is the latest 4.9.7. I’m testing this in Chrome.