Within less than 10k miles, I had front engine leak fixed already twice, and it began to leak again. Is it true, that those engines (straight six cyl) are notorious to leak and MB had some recalls ? Someone suggested that it could even be a blown head gasket, but at 40k miles ???

If the shop you are using is doing only an upper timing cover reseal (about 4 hours), you should insist that they do a complete timing cover reseal. I have done over a dozen of these and all worked out perfectly.

Lastly, I have seen head gaskets leak from the front as well as the rear.

Good luck

My MB dealer's head mechanic told me that MB has instructed them to do the timing cover work twice. If the leak reappears again, MB has instructed them to do the head gasket. Could it be the head gasket in your case? It would seem possible.