It sounds like the status of those drivers isn't getting read properly by the Duet. This could indicate a faulty DueX2, a faulty interconnect cable, or just possibly a faulty Duet. Please check the following:
When you run M122, what status is shown for drivers 5 and 6?
Are the two stepper driver chips on the DueX2 cold or hot?
Turn the power off and check that the interconnect cable is securely plugged in to both connectors.
