Firstly, I recommend you update the program to the latest version. After that, configure your antivirus and Windows Firewall to allow the software to communicate. If the problem persists, navigate to the official support page, check the I forgot my Yahoo! ID box, click Next and follow the on-screen instructions. In this way, you will recover your user ID and you will be able to sign in. If it doesn't help, check the first box from the support page and follow the provided steps in order to reset the password of your account.