This is quite unusual and there is nothing you can do about this except to report this error to the Yahoo mail technical support team.
Go to the following pagehttp://help.yahoo.com/l/us/yahoo/mail/yahoomail/forms_index.html
and from it select the form (click the page to get it), that is proper for your report.
Don't send any login information unless they ask for it (they will likely NOT ask for a password, but can ask you for your username).
This error information shows that their mail login processing scripts and possibly their database interface have been corrupted or coded incorrectly, and this is something that only they can fix, not you, the mailbox user. HTH