After getting the previous response I patiently explained that it was not possible for a server error to be the culprit since the account in question is 3 years old.
Yet, Yahoo! customer care responded to this sensible explanation with exactly the same answer (only this time from 'Nick')
Thank you for writing to Yahoo! Account Services.
We sincerely apologize for any inconvenience this may cause you. It
appears that there may have been a server error at the time you
registered for your Yahoo! account. Unfortunately your information was
not correctly saved, and you will be unable to access this account. You
are welcome to create a new Yahoo! account with a different ID. Again,
we apologize for any inconvenience.
Thank you again for contacting Yahoo! Account Services.
New and Improved Yahoo! Mail - better than ever!
Argh! But hey, I am free to create a new account! Yipee! That sounds like fun. Why didn't I think of that before?