I've just flashed my old Xtreme 5.3c to the latest iXtreme (silent edition). But after the flash, 2 of my backed up games does not work. The first one is Colin Mcrae Dirt, it just says: please insert this disc into an Xbox 360 console. And the other game is Fifa 2007 Champions league, it says "unreadable disc". Both discs worked perfectly with Xtreme 5.3c.
Any ideas what the reason for this might be?

(it works great with other backed up games)

you need to remake the backups, and properly stealth patch them.

ixtreme only boots properly stealthed backups. unstealthed backups are proven to get you banned from xbox live. the 5.3 firmware will boot unstealthed backups, which is why ixtreme exists. it will only boot safe backups that won't get you banned.

Ok, thanks for the information!