Hi Al,
I am not sure the explanation you received from UC100 has any credibility or not – It certainly seems strange to me.
You need to look at your ‘Mach3 plasma.scpost’ Post Processor as a matter of urgency – it is not preparing the Mach3 Gcode correctly (at least not as Mach3 wants to see it) and it is creating errors.
Perhaps the most important change to make is to have it place the G91.1 on a separate line (certainly not on the same line as the G90).
Try placing the G91.1 on a separate line and see if your UC100 works correctly then.
Tweakie.