Sounds like the typical MB response of not having a clue. The operation you were doing was just freeing up the movement as mentioned - the tonneau cover sticks closed and requires more pressure to open it. The hydraulic system automatically shuts down because of this. Disconnecting clears current DTCs from the ECU. Have never seen a reference to air in the system from WIS or any other source. Even the owner's manual tells you to operate the roof manually in this instance.