This is not an uncommon problem with ML's. The problem usually will set fault codes in the control unit for the central locking system (AAM- All Activity Module). The only real fix is replacing the faulty door latch assembly on the problematic door. Each door latch, including the rear tailgate, has a microswitch inside that recognizes the latch open or closed. When this goes bad, the vehicle does not know the position of the latch & tries to repeatedly lock the door, to no avail.
If your ML is still under warranty, it should be covered. This same scenario has been found in some other MB models also.