It looks like an issue crept up and some Steam Product codes vanished from the account they were redeemed on, leaving behind the error "A Steam Product code you activated has been removed from your account." in its place.
The good news is that we have already fixed this one - all it takes is a full restart of your Steam client.
The important part is to fully close Steam, don't just minimize it, then bring it back up and your code should be restored and ready to get back in the game!