Second Life gives me an error message about being "Unable to find a valid certificate" or "Unable to establish a secure connection to the login server."
This article is part of the Extended Second Life Knowledge Base that includes advanced and specialized information. This information was originally provided by Linden Lab, but is not actively maintained nor guaranteed to be accurate. Linden Lab does not certify nor assume any responsibility for this information.
See the official Second Life Knowledge Base for the most current information.
Your local network may be blocking secure logins. It's also possible that your secure login is failing due to an incorrect date/time on your computer.
Try double-clicking the clock in the Windows system tray (in the lower-right corner of the screen), and verify the date and time. Make sure it's not a year off!
Be sure a firewall, or internet security or firewall software, is not blocking Second Life access.
If you are on an office or university network, your network administrator may have secure logins disabled. In this case, you may not be able to use Second Life on that network connection.