Hi there. This happened to my S212 also. I did think was the rear door childlock on but I don't believe it was. I did need to bring it to a MB dealer in the end who told me that the lock mechanism was broken and needed to be replaced.
BTW, when I asked him how the opened the door (to get at the mechanism), the dealer said the mechanic just gave the rear door an significant almighty quick tug from the outside and it opened. Might be worth trying it. Nothing to lose.