We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
It's possible that this may be an issue if your gamertag is set up for a different region than where you purchased the code from. ie. you live in the UK and purchased the card there but your tag is set up for the US.
You should contact support for more assistance on this though as there is no official description of that error code.