I checked the fuse and interchanged the working relays for which the parts numbers match. Door locks still do not work. The liftgate opens and closes just fine. However, the 4 doors and the gas flap neither open nor close. I can feel the correct relays open and close when I push the corresponding button on the key.

Still looking for ideas. Since none of the 4 doors work, it can't be individual actuators (unless they all 4 burned out simultaneously). Are the connections to the doors electrical or vacuum? Where is a central point to start the search for the fault?

Having same problem on my Ml 430, but the problem with mine is the liftgate does not work. Are there any fixes?

This is a common problem and is caused by a failed tailgate lock actuator. Usually when they fail you can manually lock the tailgate from inside the tailgate and the key/remote will unlock it. However, due to the push/pull mechanism breaking you cannot lock it with the key/remote. Time to replace the lock actuator mechanism inside the tailgate.

A search of this forum will provide instructions on how to do this if you are so inclined to repair yourself.