Mine has been in a week now and not fixed yet.. Last time I managed to speak to anyone, they seemed to think it was one of the coolant pumps that had failed. I'm assuming these are pumps associated with ensuring the battery system doesn't overheat / freeze.
Apparently before charging will initiate, the car needs a certain signal back from both pumps, one appears to not be working hence no charging. The problem is that none of the electrical signals the garage were getting matched the wiring diagrams provided by Mercedes and the garage believes they are incorrect!
Mine was also intermittent for a while before one day, it just stopped charging. I did notice on the days it wouldn't charge that you could hear what sounded like coolant being pumped around which is never noticed before when charging.
You would hope they can get some fault information from the memory in your car if they plug it into diagnostics? Good luck, hope you get it sorted soon (and also hoping I get mine back at some point - really missing it after a week without!!).