Same problem here. After landing taxying to the terminal and differential brakes came on. No way to turn them off as you say. So not a problem with your controls as suggested. Only way to fix is to reload the aircraft.
Recently we have seen a lot of codes used to unlock our products being offered for discounted prices. Almost all of them are bought using stolen credit cards. These codes will all be blocked by our systems and you will have to try to get your money back from the seller, we are unable to assist in these matters. Do be very careful when you see a deal that is almost too good to be true, it probably is too good to be true.