Awesome. That will solve it, the 50th restart. And no, it didn't. Restart won't help, removing the game from checkout won't help, both combined won't help, other computer won't help, sending gift won't help, other browser won't help.
The error message actually switches between "There has been an internal error initializing your transaction. Please contact support for assistance." and the longer version in some occasions. There were no problems buying Dead Money the same way. Any other suggestions?