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You have done most or all of the things that should be tried initially and are to be congratulated.
But, the way it looks, the device may need either repair (or replacement if you prefer.)
There may be a misaligned position sensor, or the mechanical linkage may be defective or have slipped and be incapable of being run back to zero position.
Or everything of the position sensor and mechanical linkage sort, may turn out faultless when examined, but there should in that case be an error in the motor controller board that drives the incline.