My client is having the same issue. While I have updated him to 2.6.4 some time ago, the file on the webserver still has the 2.6.3 header. Perhaps it wasn't updated because of some change.
However, the code you suggested to remove is in both instances. Is this code a bug or something? I am concerned about removing it without have a great grasp of what it's actually doing.
After removing the same code, the issue is resolved and the order is placed immediately. Whereas, before, it was updating the checkout window to look like a 'confirmation' of the CC information, which is confusing to users because they sometimes dont see the form change.