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atz-
Can you grab the mech and force it to move a little. Like, not enough to snap it but try forcing the parallelogram to move by hand. You've double checked the B-screw stop already? If it moved too far once it can be in a position that won't allow it to move normally. Force it with your hand up the cassette a bit and see if it frees up.
Doesn't have to be on the bike, just make sure it can move.
hippy
Does the RD actually move? If the red light is on the Junction Box A it could be in fine-adjustment mode. Assuming you've already held that down for 10s or whatever then it's unlikely but possible. If the derailler is making a noise when you push the shifter button, it's not the shifter button failing it's likely trying to move but can't so either something (like my B-screw setup) is preventing it from moving or the unit is fuxxored in some other way internally, but that seems unlikely given not crashy crashy (I assume, not been following conversation).