The Lowrider2 no longer has a “Full Graphic Smart Controller, Big” attached to the Rambo 1.4a. It went bad and I replaced it with a 7" touch screen attached to a Raspberry Pi, which in turn is connected to the Rambo 1.4a. No error was shown on the touch screen, or on Repetier-Server’s web interface, merely that the firmware had issued a Stop.
I tried the " G-Code Q’n’dirty toolpath simulator" you suggested. It complained at the beginning that it did not understand M117, but that was at the very beginning and gcode was well along making cuts before the Stop. I don’t remember the exact numbers, but they were all positive. X was between 0 and 20. Y was probably less than 300. Z was between 10 and 20. I ran the Fusion 360 simulator for the gcode, which contained only the 2D Contour operation. It completed successfully. I looked for a simulator that was Marlin specific, but did not find one.
jeffeb3 mentioned my machine is larger than the max values in the firmware, and that caused the problem when homing. I was thinking this was not the problem, as the gcode had already traversed around the piece at least once, and was making its second or third pass (each cutting another 6mm deeper).
I would try rerunning the code in the morning, unfortunately the power company is cutting our neighborhoods power for 8 hours. I will try tonight.
If the problem occurs “in the air”, I could try rebuilding the Marlin firmware with the actual machine values and see whether it resolves the problem. Does Repetier-Server have a plugin that gives better error messages for Marlin firmware? Any other techniques for getting a better error message? I will also look in the gcode Console. Maybe something got dumped there that I did not notice. Thank you for your suggestions. I am SO close.