Try using a different memory card with your action replay. I went through exactly what you did, but ended up taking the AR back because it didn't work for me. Then I read here (
http://forums.xbox-s...ST&f=44&t=87183) that certain memory cards unsign the app which is why you get the error 21. I believe the AR memory card does that, as none of the various hacked saves worked for me. When I rigged up my own action replay after doing some more research, I managed to get it to work even though the card I was using was a Gamester (i.e. very cheap 3rd party card). There is a tutorial about that written by Vintage Guitar that you should check out, as it deals with your issue as well. Many people say that the MS brand memory cards will work for sure, so you might want to pick one up or borrow one from a friend. Hope this helps.