I am having the exact same problems as the poster above. I get this identical message concerning the "credentials" - what are they supposed to be so I can start accessing my ntlworld address emails again? I don't know what has happened, and it's been going on for weeks now.
Sorry to hear of the email issues experienced, we appreciate you taking the time to raise this via the forums and welcome to the community.
In instances where this error message is received, resetting the password for the email address usually resolves the issue. Details on how to do this can be found here.
If this doesn't resolve the issue for you, please let us know and we can assist further.