Robban replied the topic: OAutException(#3)App must be on whitelist
I have exactly the same problem and it started yeasterday after 9 months without any problem!
I can add my facebook acount in the backend and it gives me the state "OK". But accessing it via a channel - the channel prompt me "OAuthException: (#3) App must be on whitelist"...
Please investigate and prioritase this issue asap!