If you having troubles signing into Skype For Business desktop app on Windows 10 and it is coming up with a certificate error, then it’s because auto detect proxy settings is ticked in the LAN setting. Windows 10 works differently with the Auto detect, so if it is ticked Skype won’t sign in.
To change setting go to
Internet options (In control Panel or the tools section of IE, or settings in Chrome)
Go to the Connections Tab
Click LAN Settings
And untick the Automatically detect settings