Could be:
1. Bad cable
2. Connectors broken, not fully seated, shorted
3. Faulty MU1000.
4. One or more of the MU1000 Layer buttons is stuck or malfunctioning.
(You DO know that you can over-ride the layer selections on the MU1000 by pushing those buttons on the right hand side of the meter unit, right?)
Do all the other channel meters in the mixer behave normally? Can you affect their behavior with the various settings in the Meter Menu? If so, the problem is 1, 2, 3, or 4.
CaptDan