the symptom is the motion controller displays an error that says a "CPU watchdog failure has occured. This is generally a result of EMI".
That tells you that the motion controller is susceptible to EMI. A company I worked for years ago owned the design of a PLC that would watchdog all the time due to transient electrical events.
That was when we got serious about "designing for EMI". When a piece of equipment is not designed to withstand appropriate levels of EMI they will often suggest that the problem is in the grounding.
Do the specifications on the unit state their EMI immunity levels? If not then I would put it back on them.
EMI can enter a device in many different ways. Either conducted or radiated. Cables connected to the device provide a common entry point. The longer the cable the better "antenna" it makes (generally speaking).
Any chance that hand held radios are ever used in proximity to the equipment? The possibilities are many when it comes to EMI. I agree an electrician should not be the one responsible for troubleshooting EMI issues (not saying an electrician should not attempt it, just not be held responsible). There are lots of EMC consultants out there for hire. ( I say this but be cautious)...
We once had a field issue where the manufacturer of a piece of equipment was telling us that our grounds to their equipment were insufficient. They hired an EMC consultant who wrote up a nice 7 page document "proving" our grounds were bad ( too high of a resistance).
It turned out that he took resistance measurements with the power on to the equipment! I was very surprised by this.
I then spent several days on site monitoring via oscilloscope with 2 engineers from that company. When we observed a transient event they stated " look there is the transient caused by your poor grounds". I noted that the "transient" was not bi-polar which I commonly see when it is a noise event.
After capturing a few more events we found that their equipment had an intermittent open, on a wire in a cable, that was part of their motion control!
Best of luck to you.