Actually found the issue by simple debugging. It was in the “Instructions” section. I rewrote it and it worked. Probably used too many code words or JSON words. Don’t know…
Just had the same issue for the first time. Last night I downloaded the android app and signed in, then this occured on desktop.
I had to open my android app and sign out, then sign out from desktop and back in. Then it worked. Seemed to streuggle with 2x active sign ins on different devices.
Probably something to do with account sharing prevention 
This solved the issue for me
You’ll need to turn off any URL blockers or extensions that you currently have enabled, which are preventing ChatGPT from loading correctly. (I work with Chrome)
Clearing cache/application data for the site solved it to me.
Im facing this issue with 4-0 now. There was a problem preparing your chat. Please refresh the page and try again.
I encountered the same issue, but when I deleted all my ChatGPT conversations, it worked again.
This is most likely just a temporary issue. I don’t recommend deleting all conversations unless it’s the last available option.