Quick fix to get people going again. (worked for me at least)
Exit GeForce Experience
Open a run dialog and go to %localappdata%NVIDIA
You will find a GFExperience.exeUrl folder here, open it and then the version folder.
Rename or delete the user.config file.
Restart GFE (do not try to log in)
For anyone trying to use Gamestream, you should be able to find your computer through "settings" in the hub. You will be prompted to enter a code on your computer, put that in and you should be good to go.
Keep an eye on the Nvidia forums for an update.