Code 400: unable to read file - doesn't appear to be gcode (with untouched gcode that I ran successfully last week)

Machines (Original/PRO)
2

Just an guess:

You are including N-Codes (basically line numbers). I don’t know if the Onefinity system understands them or not. My CAD/CAM software doesn’t produce them.

This may help: N-code instead of G code - #2 by wolfkabal