@YouTubeIndia what are u doing?? Yesterday I uploaded 4th video on my channel without any custom thumbnail. The error is not getting solved. This is ruining my channel and my videos too. Everything is perfect the size and format but still json error persists !!! Do something!!
Sorry you're still having trouble. Since, you're on Chrome try unchecking "block third party cookies" in the browser settings to see if that helps? Here's how: https://goo.gl/WqhyRm . Let us know how it goes.