App Store error "An SSL error has occurred and a secure connection to the server cannot be made."

Check that your system time is set correctly. Remove all add-on anti-malware, anti-virus and security products per the vendor instructions, reboot, and try again. Alternatively, reboot into Safe Mode and try again. If that fails, check your network configuration; look for some router or gateway device that's attempting to intercept network connections, or that's attempting to redirect connections. If it's not the local time getting skewed, it's usually some add-on tool that's stepping on connections. Either within the host itself, or within the local network connection path.