The "error 999" code is often caused by unusually high activity from either your computer, mobile device, or IP address to specific Yahoo! pages.

While this type of error is usually temporary, the troubleshooting steps below can help resolve the error:

1. Try accessing Yahoo! from a different computer or different Internet connection.
2. Scan your computer for spyware and viruses -- Spyware programs can change your system settings, serve pop-ups, record your surfing habits, or display advertisements over websites you visit. Other spyware programs and certain computer viruses will run separate programs on your computer for a variety of purposes, sometimes slowing your computer down in the process, and even preventing you from accessing certain Internet sites such as Yahoo!.
3. Enable cookies on your web browser -- Also make sure that your web browser is enabled to accept cookies from Yahoo!. Deleting your cookies can also contribute to your receiving this error. For information, please visit our help article about enabling cookies.
4. Does your household or office have multiple computers accessing Yahoo! through a shared Internet connection? -- If so, you may collectively have to moderate the amount of access you have during a given time frame, so you won't exceed the limit of access from a single IP.
5. Do you use a third-party software program or application to access Yahoo!? -- If so, please keep in mind that third-party programs are not supported by Yahoo! and may contribute to you receiving this error. For example, certain plug-ins, depending on how they are configured and the level of activity in your groups, may lead to your receiving the 999 error.