I am having the same exact problem!
Please could you send me a similar message to try to resolve the issue for my account?
Thanks in advance.
Hi, the step that ultimately fixed my problem was to have different passwords for mybt and your email account. The system should let you use the same password but there is a little big.
Just go to bt.com, go to log in and click forgotten password. It will give time the option to reset either password.
Hope that helps.