Wawa:
I think I see that you have connected relay board ground to 74HC595 and Arduino ground.
That makes them NOT opto isolated.Relay supply connects to JD-VCC (jumper removed) and relay ground.
74HC595 connects to relay VCC and relay inputs. NOT to relay ground.This might have nothing to do with the problem you're having.
Leo..
Relay ground is not connected to Arduino ground. That would indeed make them not isolated. Relays have JD-VCC & ground from second power source, and VCC + signal from Arduino.
I also tried to power both the relays and the shift registers from the second power source. Even though it wouldn't serve a purpose in my opinion, because the opto-isolation of the relays should be enough. And it also didn't improve my situation.
This morning I removed all the wiring. Only attached one shift register and one relay. Then I put aluminum-tape around the 24V cables, alu-tape covering the relays, grouped all wires between relays and shift registers and wrapped them in alu-foil. Put the shift register in a plastic bag and wrapped it with alu-tape.
All this shielding, and it gives me the exact same problem. This is getting ridiculous 