First of, Quall the app is named "Qwix" without the "u"...
Weasel have your previous backups worked perfectly? And when you say backuped to CD I do hope you wrote wrong and meant DVD.
Here is how I did my perfect backup of Kotor2 a long time ago,
Popped in the DVD into the xbox.
Started FlashFXP and FTP'd all content to a folder named "kotor2" on my desktop.
Used Qwix to create a iso of that directory (yes Qwix can do that
).
Burned the ISO with DVDDecrypter and played the game.
Dirty Disc Error may happen if the xbox can not read the DVD properly due to bad media, bad burn or some fault in the iso creation process.