me too - it was playing fine yesterday morning on ipad then error 21 last night. If we try to load it thru the tv it says 'content request not available'.
Has it been pulled from Now TV with no warning? Very unhappy kids - Now TV pass about to be cancelled.
Came looking for a solution for this same problem, cant get adventure time to work either on my android phone or ps4 app and I specifically paid for NOW TV for adventure time 😞 apparently error 21 means its unavailable and not an error with Now tv itself but struggled to find much more info than that