OK, you have the 5 V going to "Vin". It should go to "5V" on the Nano. "Vin" is the input to the on-board regulator, which is useless.The resistors and capacitors should be at the start of the LED strips. All the wires from one part to another should be bundled tightly together so that no open loops are formed.
I suspect the correct power supply connection will be the fixer.
If you have a regulated 5V supply, you should just connect to the 5V pin / node.Vin has a regulator which supplies 5V to the 5V pin / node. The regulator needs >7V to properly regulate down to 5V. Putting 5V in will get you less than 5V out, leading to unstable operation.
Am I somehow acting as a sink for EMI or RFI?
Might it be worthwhile to try adding a 0.1µF capacitor across the opto input and ground?
Depends what you call ground. I would try something across the LED side of the opto.Looking at the graph I would have thought that the detection circuit is not feeding the input side of the opto with enough current to turn it on hard enough. Again capacitors on what ever that detection circuit is.
Why do you have a 10K resistor across the capacitor? That just reduces the effect of the capacitor and increases the loading on the signal source.
That diagram makes no sense at all. You have connected the -5V output as a signal ground and the -12V as an other sort of ground symbol.I simply don't understand it sorry.
-5V and -12V were my attempt to indicate that they are separate grounds.