Hello Stefan,
I was reading your answer about a LiveCD for Debian that is accessible.
You answered by giving the info about Knoppix, but Knoppix is NOT Debian. It is not Ubuntu either, it's just Knoppix.
You cannot successfully use Debian files in Knoppix or convert the system to a Debian system.
Do you know if any of the LiveCds for Debian are accessible?
Debian 8 is accessible from a regular installer by pressing the letter s during installation.
David