[weblacky]

Meanwhile I have examined the backplane. It contains passive components only, except the EEPROM. I see no damage on the PCB. There are no elcaps. Contacts are cleaned with a contact cleaner spray. Measured the voltage when the CPU board and gfx board are plugged in. The voltages are okay.

Unfortunately I have no 2nd machine to test the CPU board and the gfx board.

Any idea?

I don’t have any personal experience with this machine. I own one, but I’ve never even started it because I have other things to get to. But since it appears to be a brutally basic connection problem (cannot even detect the presence of the card), I would assume you have a cracked solder joint in one or more places in your backplane or connector. So as was suggested I would go back to your backplane and very carefully use a multimeter to start toning out connections, and see if you can find a higher resistance connection or an intermittent connection that might explain this.