Knoppix is doing its best autodetecting the PC's hardware and in many cases it gets it right. In this case I think it is setting the video resolution & refresh rates beyond the capabilities of the monitor.

Knoppix boots with a splash screen. At that splash screen you can enter codes which will force Knoppix to do certan things. Theses codes are known as Cheat Codes. You should try the ones that involve the video, such as:
knoppix vga=normal
knoppix vga=normal screen=800x600 xvrefresh=60