Selector does not home on MMU2S startup + calibration does not work  

Selector does not home on MMU2S startup + calibration does not work

Hello.

I assembled the MMU2S unit for my MK3S but cannot get it working properly:

When I power on the printer I understand that the MMU2 is supposed to home itself. On videos I've seen first the idler homes itself by rotating until it bumps into the "endstop" and does that multiple times (using stallguard of the TMCs I guess). In some videos the homing is quite loud (on mine too), in some others actually pretty quiet. Then the selector moves to the right as far as possible, backs up and bumps to the right a few times reducing the travel distance each time.

Unfortunately my MMU2S only executes the first part: homing the idler. The selector does not move at all when I power on the printer/MMU.
When I press the right button on the MMU afterwards (after the homing procedure stopped) it starts the (correct) homing procedure: homing the idler and then homing the selector.

Is there any way to find out why it does not home the selector when powering on? Also it seems to home the idler pretty often (sometimes even every time before moving the selector to the left/right with the buttons).

I'm on the latest firmware for MMU2S and MK3S. I re-flashed the firmware multiple times. In the support menu it shows the correct version for the MMU. I also did factory reset the MMU from the service menu.

I also checked the FINDA in the "sensor info" menu. It works as expected and changes when inserting/removing filament in the selector.

Is the selector supposed to home itself when filament is loaded when powering on? Or does it only home when the FINDA tells it that no filament is loaded?

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Another problem is that I cannot calibrate the PTFE tube length in the service menu:

I followed the instructions and wanted to calibrate the first filament. When I start calibration the idler and selector home themselves, then the idler rotates so the bearing is over the first pulley so it can grab the filament. When I push in the filament it grabs it and starts accelerating after a short while and tries to push it to the hotend.
In the manual it says to open the idler on the extruder (by removing the screw and tilting it open). This doesn't work for the MMU2S, because of the chimney which prevents tilting it. So I removed the idler (by completely removing the M3x40 screw). Now I'm able to see the bondtech gear.
Unfortunately the filament is down way too low and rams into the hotend. So I tried to retract the filament in the calibration process. But this does not work because after pushing the filament down to the hotend the MMU idler rotates slightly so it loses the grip on the filament. When pressing the left/right buttons afterwards the pulleys rotate, but they do not move the filament because the idler bearing doesn't press on them. Is this supposed to work like this? How could one move the filament with the buttons then in the calibration process?

Also is this the correct way to calibrate the MMU2S (not MMU2!)? Am I supposed to remove the whole idler on the hotend to see the filament position on the bondtech gears? The guide shows screenshots from an MK3 (not MK3S), so I cannot tell the correct way for the MK3s.

Is the filament sensor on the hotend involved in the calibration process? - Due to the disassembled idler the filament sensor on the hotend cannot trigger. Do I have to disassemble and reassemble the idler after every calibration step?

I hope someone can guide me in the right direction so I can finally start printing with the MMU2S.

Thanks,
Chris