Thank you and good day
It is only PAL Gamecubes that aren't compatible!
Mad Catz and a few others released a Gamecube S-Video cable at the launch of the European Gamecube thinking that their cables were universal. Currys and many other chains sold them and continued to sell them after release, despite the cables not working on PAL machines.
I tried one out they definately don't work on PAL Gamecubes.
Only RGB, Digital HDTV and RF Cables are compatible.