I've been dealing with a horizontal banding issue. We have one of the early UM2's with the rear fan pin on the end of the board rather than in the middle. All belts are well tensioned and all axes are greased or oiled.
Initially this journey started with lots of underextrusion problems, we were printing ABS and PLA (PLA/PHA) with the same nozzle. Despite atomic cleaning, there was some material burnt to the inside and causing insulation. We constantly had to increase temp to minimise underextrusion. Robert's great extruder upgrade helped, but it it was fully solved with a ne