Cheatin uh message when I click on customize button

I keep getting the message Cheatin uh when I click on the customize button. Anyone know why that is? It has only happened since the last wordpress upgrade.
I also can't seem to preview posts either - as superadmin no matter which site I was on I could always see a preview of pages/posts and now I can't. I get the same cheatin uh message.

Frustrating!