Welcome to the forum.
Have you created at BT ID for yourself?
If not, uninstall the MY BT app, create your ID then reinstall. Check that no trace of the initial installation of the my bt app remains on your sd card on your device[s].
Let know how you get on................
In that case I'm wondering if there is some element/aspect of your account that has not been initially correctly provisioned.
That can only be checked by bt themselves, getting that information over the telephone would I imagine be a living nightmare, I am therefore asking one of the mod team to check that out by working on the details of your account with you.
in my experience of using the app it has been a bit flaky from time to time, so it might just be app functionality.
Edit:- Ref your other post/thread, have those 'old' bt accounts been removed or is there still a trace of them?