"Unable to find a valid certificate" or "Unable to establish a secure connection to the login server"
Your local network may be blocking secure logins. It's also possible (but comparatively rare) that your secure login is failing due to an incorrect date/time on your computer.
To verify, double-clicking the clock in the Windows taskbar (usually in the lower-right corner of the screen). On a Mac, check your Date & Time. Make sure it's not a year off! Most modern computers can set their time automatically, including daylight savings.
Be sure a firewall or Internet security isn't blocking Second Life access.
If you're 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.
If you are on an office or university network that uses HTTPS inspection tools such as Astaro Security Gateway or Kaspersky Internet Security, you likely have your organization's Proxy CA installed on your regular browser. Export the CA to a PEM file and add it add it to the file CA.pem in the folder app_settings in the folder where you installed the Second Life viewer).
Computer's clock is set incorrectly
The full error reads:
Unable to connect to Second Life. Often this means that your computer's clock is set incorrectly. Please go to Control Panels and make sure the time and date are set correctly.
This message has been around a long time and can be misleading. It refers to time-based sensitivities with security certificates, but normally isn't a problem, as you can test for yourself by deliberately setting your computer to the wrong time and trying to login. It should work.
If you're absolutely certain your time is set correctly, you haven't changed your system setup, and Second Life was working fine moments before, it's likely a transient problem with our servers. Check the Grid Status Reports page for news and try to login periodically — often all it takes is patience while we fix Second Life. If you continue to get this problem after logins work reliably again for other Residents, contact Support.
Second Life fails to load and displays an error regarding DINPUT8.DLL
This usually means that Microsoft's DirectX is not installed properly, or is an older version. To fix this, install DirectX 9.0. You can download DirectX at Microsoft's website.