The Digital realm can be quite tricky especially when it involves HDMI connections. There are instances where the unit expects to receive the test tones via the HDMI inputs due to the way the technology behaves. The typical way to resolve this would be to change the active input on the MRXx40/AVM 70/90 model to Analog 1 or Streaming before connecting to ARC and setting up the measurement.

If this does not work you may be able to resolve the issue with a power cycle of the unit by disconnecting the power cord and pressing the front power button ~10 times.