All right I got an XBox that was sent back to me by a customer. The XBox is a version 1.1 with an X2.3 (red PCB) I installed some time ago.
The XBox boots fine when the chip is off. When the chip is on, it boots up, I see the X2 text than it gos to its error screen and gives me the error 21.
Chip is flashed with X2 4981 (if my records are right) on both banks 1 and 2. Tried every thing I can think of short of reflashing / replacing the chip. Comes up with this error message no matter what. With no disk in, with an original game disk, back up game copy, and even EvoX on a CD.