I have managed to reduce the non-working valves number to 2. Still there is the issue with high voltage on non-actuated valves, even higher when no cable attached to the two respective IO port.
I have unfolded a secondary 2-wire cable just to test the two non working valves some more. I have left the return path on the the old cable and made various combinations between the 2 IO ports, the 2 wires in the secondary test cable and the 2 valves. Results: both valves working on only one of the ports. I am now stuck. Help!