This is a coding error, obviously... But... Here's how I fixed mine... Perhaps it'll work for you too...
When at the log in screen, you'll see a box for your key and a "Modify" button. DO NOT press that button. Just log in, let it ask you for the key with a popup.
After that, exit the game. Don't go back to single player mode, don't do anything. Just exit the game.
When you open the game and log in again, your key should be saved.
Cheers.