Everytime I try to preview a theme or click to customize of the current theme I get the "Cheatin, uh?" error. When clicking to customize I can see the options window and in less than a second the error is shown.
Looking for that problem I have found out many people is getting this error when uploading new images to the media gallery, I can upload images just fine.
I'm using WordPress 3.4.2. I have tried disabling all plugins and with serveral admin users and blogs.