A thousand pardons if I'm wrong, but my impression of the OP is that he might NOT be much of a hands on guy. I would hate to see him start throwing parts at his bike to fix it! This bike needs some EXPERT troubleshooting. NOT suggestions saying he has a blown controller, or worse, an entire motor.
I would encourage him to learn enough about his bike/motor to enable him to perform this expert troubleshooting, or lacking that, find a local shop that can/is willing to give him a hand.
I own an Ultra, and had no idea they had a 2 year warranty. Love the idea of contacting Bafang with the hopes they'll be able to guide him through this process.
I really hope Mobe1957 will update this post as he works his way to a solution. That way we can all learn something maybe? -Al