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Sometimes after an update, (depending on what installer you used), UnleashX refers to the dashboard in a different directory than you old one. Check the location in the config.Xml, and check if the .Xbe is there. if not, change the directory in the config.xml to the location of your MS dash (look for Xboxdash.xbe)
Ok, maybe i gave a wrong answer... an Error code 21 refers to a missing or invalid executable the system is trying to execute. (on stock Xbox's its the MS dash...) and if it frezze's at the loading screen, re-install your mod. then replace just the .Xbe and the rest EXCEPT: the config.xml leaving your original from your older dashboard...