Well, Error 21 could be a lot of things but a large majority of the time it is due to the dashboard having a damaged file or maybe even a missing one. First try an original XBOX game disc that has live capability. Assuming you have a live account, launch XBL and a dashboard update should fix your problem. If not, then yes, a chip can help you in fixing your problem. It's of course a more expensive answer but I'd suggest trying the above first.
