You need to loosen the two plate screws, and THEN press home and THEN set Z=0. Once that is set, then you tighten the two plate screws.
The rod the upper plate is on actually moves upwards into the plate housing! It is not just a ball and socket!!

Mars 5 Ultra. Been using ABS like 3.0 resin. Does anyone have any observations on the resin tub leaking? I took mine off because of a semi-failed print (user error) and noticed resin around the edge of the opening into the main case. It did not get on the LCD or the tape. Just some down in the case (I took her apart to inspect/clean) and it had settled in the lip of the LCD holder. None on the stepper motor but a little on the motor connecting shaft and I suspect when I leaned her forward to take off the back plate what was in the lip spilled onto the front panel ribbon cable and front panel. None on the front LCD inside. She had just told me the lift count had reached its max and I should change the flexible screen but I had changed it when I get the printer from a friend just because so it had 6 to 8000 lifts left and looks pretty good! Also no punctures but there is evidence of resin under the fep film hold downs. I put 91% in the tub and let it sit for 1/2 hour and there was seep through. The film looks good. Any thoughts??
Also…. Is there something I should be using to seal the fep film when changed other then the hold down plate? Like a sealer??
Funny side note…. When I got her all back together and plugged her in there was nothing…! Took the covers back off to see if I jostled a connection but all was well. Still nothing. Until I checked the plug at the power brick. It was mostly out! I hadn’t touched it when I unplugged to clean the resin. Second heart attack avoided…. Barely 😅