En Masse posted that it is aware of the situation and is working to resolve it. The studio says that players who purchased a physical copy of the game and signed up for a subscription before entering their game code triggered this problem. However, we've also had reports of players who have encountered this error when dealing with digital copies.
Affected players should now be able to log in to TERA while En Masse works on fixing the error.
[Thanks to Andrew, Schippie, and Alex for the tip!]
[Update: En Masse has contacted us to note that the 2000 players have been affected by the "insufficient remaining subscription time" issue, not the "immediate billing" error. En Masse says that these are two separate issues that the studio is actively working to resolve as fast as possible. We've updated this article accordingly.]