Here's an idea: Multiple install and reinstalls do not clean up after themselves in the the registry. If you're careful and you know very well what you're doing, uninstall first, then try Regedit and find the Googleearth entry in HKEY_CURRENT-USER --> SOFTWARE --> GOOGLE --> GOOGLE EARTH PLUS and delete it. Then reinstall.
I'm suggesting this because I think it's what made it work for me. I kept getting authentication errors for a few days. Uninstall/ reinstall didn't work. Re-download and install, didn't work. I tried all the suggestions I coud find, including: Disabling antivirus, disabling popup blocker, disabling proxy setting in IE, disabling firewall altogether, rebooting the wireless router. None of them worked! Until I deleted that one entry in the registry.
Let us know what happens. And please remember, you touch registry at your own risk! Backup before changing.
I wish GE was a little more explicit as to the cause and a little more careful about cleaning up.