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so far, there is no much matching errors to this number. but i think it refers to your alternate email address? anyways, try this option but i can't guarantee if it will work.
sign in with the account and click "Forgot My Password". see if you can change the password. or else, enter a wrong password until it will lock your account for 24hours. then do not attempt to touch it between 24hours. after the time lock limit passed, try to log in with the account using the right password this time. one attempt if it fails, stop and change the password. be sure you can provide the full info of that account in order to receive a new password. and or, sign in with that account on the other services of yahoo that you don't enter as often, such as Yahoo! Auction and see if it will log you in without any error.
lastly, before doing everything that i had mentioned, delete your browsing history, delete temporary internet files and delete your cookies. clean your cache. then restart your PC before signing again. i think it will help by all the process i said above.