The SSR switches on/off, though sometimes gets caught in the unknown state, meaning the LED light in this video sort of flickers unpleasantly. I tried to catch it in the unknown state, of course it doesn't do it when the camera is on.

In the video I'm toggling Flood in mach3.

https://youtu.be/sjDd0g25Hrc

From the first video, note that output 3- on the MX3660 is connected to input 2 on the relay board (output 3+ is not connect to 5v+, only Output 3- is connected to input 2, this is visible at the end of the first video). The input 2 LED is lighting on the relay board despite the Output 3 pins being turned off in Mach3.

I haven't had time yet this evening to try the resistor between the ground and the input. My wife is ill, but once i get my 3 year old to bed I'll try to get out there.

I doubt a ground loop is an issue, I don't have any steppers or accessories connected as I am bench testing, also, my system was dead reliable before. I've added this SSR board, and the additional BOB (nothing is connected to it). But changes being changes, I suppose it's possible.