Strange issue! You should be able to log into the app using the same credentials as what you're using to log into your MyBT profile on our website. Have you spoken with anyone since you posted this? If you are still no further forward post back and we will give you a hand from here.
Sorted now. Took a few goes via the BT help chat people. I have had loads of issues with the App caused initially by the fact that my email was associated with a Business account - I have never had one. Had to have my original profile deleted and a new one created. All was well for a week and then this happened. All now ok - for the moment!