Looks like you've got a blockage somewhere in your evaporative emission system. I can't find anything specific to Mercedes at the moment, but there's a generic description at
http://www.aa1car.com/library/evap_system.htm . When you fill the tank with liquid, the vapors have to go somewhere, and if they can't get out through the vent system, they have to come back up the fill pipe, which results in the symptoms you're seeing.
The blockage could be could be caused by one of the controls not working properly, or it could be a kinked or plugged hose or pipe, or something in the charcoal cannister itself. It might be easy to find, or it might be a pain. If you've got a good shop I'd be inclined to take it there and let them find it.