The default release for Knoppix 7.4.2 has been wheezy, but some days ago wheezy has changed from stable state to oldstable and will no longer be supported. Now the release "jessie" is stable.

To solve the error within Knoppix 7.4.2 and 7.5 (CeBIT version) you need a new file '/etc/apt/apt.conf' containing
Code:
APT::Default-Release "jessie";
After this you have to do
Code:
su
aptitude update
apt-key update
Please don't try upgrade or dist-upgrade with Knoppix!