Having the exact same issue, just started tonight. Nothing different in my app to the way it was 2 days ago. Just a totally unhelpful error code 15 (serverRejectedRequest). Can see my container fine via CloudKit console but the app is not able to connect. Evening of development lost, very annoying.

Same for me.

I'll add my name to the list of sufferers. Occurs with regularity, never seems to be fixed via the same route twice, can result in hours of lost development time. I'll well capable of wasting my precious time with my own technical failings, I really don't need Apple to add this to the mix!