Credentials means email address and password. That's all you need to sign in to your mailbox.
Heads up that the sign in webpage has reverted back to the normal one for all migrated mailboxes to the new look mail platform but your browser may still be retaining the login details / cookies for the temporary sign in webpage. So clear the browser cache and cookies and delete any saved login for the former sign in webpage.
Select here: Sign in to TalkTalk Mail
Enter your full TalkTalk Mail email address, select Continue and enter the password, select Sign in. Assuming you get signed in OK then save the sign in as a replacement bookmark.
If you still see an error message about 'Credentials' when signed in then it may be that you have added a secondary email account to the primary login and it's the secondary email account that isn't connecting. This is because the encrypted password is not migrated to the new mailbox and in any event you've changed the primary mailbox password so the secondary account passwords needs re-entering and saving.
Let me know if this situation applies to your primary mailbox. If you don't know how to update the secondary account password I'll guide you.
Screenshots are always useful to understand what's happening so that I can see what you see. Redact any personally identifiable data before uploading here using the camera icon above this reply area.
