My XBOX couldn't read the game since a few days and I looked at the disc.
There I saw a round scratch all over the CD and I'm sure that I did not scratch the disc somehow.
I searched the Internet for the issue and found out that the XBOX 360 laser could've done the scratch and that a repair in the limited warranty time (I purchased the device in August 2013) is cost-free, because it's a rellated issue.
My questions about this:
Is the issue covered by the limited warranty?
Who replaces my game for me? It's already not useable anymore.