We were having this same issue. I contacted both QBO support and CNB support and was put on a list to be notified once the issue is resolved. However, TODAY after two weeks I was able to reconnect our CNB accounts to QBO- so I suggest trying right now! We didn't get any notices about the investigation (which we reported and were told we would receive), but I figured we wouldn't hear anything anyways so I've tried to reconnect to CNB every day hoping we would get lucky (and we did!).

In QBO, I copied and pasted the full URL of the CNB website login page to the Link Account section and then chose the option with the CNB logo. I also had the CNB website login page open (not signed in). Not sure if this will work for you now but worth a shot.