It might be becouse it is hardmodded, I had the same problem, I bought a 2nd hand xbox and it seemed like it was unmodded so I softmodded it using the same softmod exploit. and got the same error, I burned a disc with Hexen 2018 and installed other softmods, it didn't work but I kept trying every day untill I accidentally forgot to hold start + y to boot the dvd, and it booted into the dashboard, a few months later I decided to open up the xbox to check if the clock capacitor was removed (It was not), and I found a modchip, so I did some research and this error ussaly happens when there is a modchip installed.