Hi Catalin,
If you disconnect all of the wiring for all of your valves from the OS, what voltage measurements do you get at each of the 8 stations when they’re off and again when they’re on? Something like the table below. It’s not clear to me from what you’ve described above if you’ve tried that or not. It could help you narrow the problem down to your wiring/valves or the OS itself.
1 2 3 4 5 6 7 8
Off VAC 0 0 0 0 0 0 0 0
On VAC 24 24 24 24 24 24 24 24
Mike