ios10 iPad stuck on waiting for approval

Just upgraded my iPad air 2 to IOS 10 and it is stuck on "go to one of your other devices signed into iCloud to approve this iPad" with the message "waiting for approval".

None of my other devices have received any kind of message or code to approve it.

I remember this happening a couple of years back and the fix was to turn off iCloud keychain in all other devices and effectively start over with one device. However, I can't turn off iCloud keychain. Whatever I do, it just comes back on again - this has happened on my phone and a second iPad.

Additionally, when checking iCloud account settings on my Mac, this iPad shows as connected and approved. If so why am I being asked to approve it?

Slowly going up the wall with this...