Oh, another M$ account creation trick. When they discontinued Skype they said you could just log into Teams with your Skype credentials. Not so! That just forces you to enter an email address (that's not already associated with a M$ account) and guess what.. now you have ANOTHER M$ account. After this the Skype credentials are invalidated.
Heh, except they weren't (they said they would be). I can now log into M$ with both my old Skype userid and with the email address it made me enter. Not quite clear on whether this is two accounts or just aliases for one.