I have exactly this problem as well. I can log into my BT account, but I get the same message. I can see my bills, but I can't run the broadband checker (I'm trying to raise an issue with that), but it says it can't find the account. Also the app doesn't work, but it may be the same issue. I have had a BT account before, and I made a new email address just for this, so it should be unique though.
Hi @jonowi,
Welcome to the community and thanks for posting. I'm sorry you're getting an error in your MyBT account, we'll be happy to look into that. I'll send you a private message so you can get in touch with the community moderation team.
Thanks
Neil