To use Halo2 on a modded xbox using a debug BIOS, you should use EvolutionX to launch Halo2 (UnleashX makes Halo2 act buggy), and make sure TSR is set to disabled. Make sure to reboot.
Then, if you've ever used xbox live with Halo2, you'll need to kill the Auto-update by going into E/TDATA/4d530064/ and rename the $u folder to something like 1$u. You should also rename the $c folder.
And if it still doesn't work, delete everything in the X, Y, and Z drives, start Halo2 without the Debug BIOS, wait about 10 minutes so it caches all the data, and then reboot and use Halo2 with the debug BIOS.