Warning: array_key_exists()

Same here.. Yes, disabling photon makes the error go away, but it needs to be fixed so we can use Photon.

I agree. Kind of like fixing a broken car by not driving it ??

Same problem on my site.

EDIT:
I have found this solution:
https://www.ads-software.com/support/topic/error-in-a-post?replies=4

I had to disable photon as well otherwise posts keep showing the same error message as others have said here.

Thanks for the reports.

We have fixed this issue, and it’ll be part of the next Jetpack release.