No image on S-Video and RGB output, only composite works

My GameCube has been sitting idle for a year or so in a dry and clean environment. Powered it on and at first nothing happened, no sound, no image, nothing.

After blowing into the air intake, it seemed to be booting and I got sound via the RGB cable but no image. Tested an S-Video cable with the same result.
Then I used the yellow connector from that same cable that carries composite and now the image worked fine.

I know the cables to be working as they are playing fine with my SNES.

Is my motherboard fried? I already cleaned the analogue connector with alcohol without any improvements.