I have just been having a long chat with JAZZCNC on the phone, and we have narrowed the problem down to the MDI.

If the MDI is highlighted and then you click the X-Y Probe or the NS button, Mach3 will crash.

But if you have the MSI highlighted and then click off the MDI and then the X-Y Probe or NS button, Mach3 will not crash.

JAZZCNC is also about to repeat this crash on his own computer(s).

I can confirm that Mach3 R3.043.006 will not crash while doing this, but any newer versions will crash.

I am sure JAZZCNC will explain in more detail when he is next here.

Best Regards.