There is a possibility that your account has been hacked . try changing the password of your account and update new email address as a recovery and change the security questions also in the account .
You can try clicking on forget the password option and then you will see the recovery phone number in it . Do you still have the phone number ending with last 2 digits 85 ..?
try logging out your AOL Gold and sign in again and send again to the group . It should send to all the people you have in the group .
Let me know If it works ..
Are you using your yahoo account direct on web broswer or on third party application such as Outlook, Thunderbird ??
If you are using on Web Browser , delete the cookies and history and try again sign out and sign in to your yahoo email . It will work