Glad you found the problem. If I may, I'd like to offer a couple of observations. The tank referred to as the 'overflow' or 'expansion' tank, to which this hose connects (# 6 I believe?) does not function, as I understand it, like a normal overflow tank, as in most cars with crossflow radiators.
The principle behind the tank in the MB design is to act as a pressurized resevoir for trapping air in the coolant. It's mounted so that the level line of the coolant in the tank is about the height of the top of the upper radiator tank. Notice that the filling of the system is from the bottom of the radiator and coolant passages in the engine (hose # 5). This method allows entrapped air to rise up and eventually end up in the top portion of the filling tank, which is under pressure.
The actual overflow, in case of over-expansion from excess pressure, would be out that hose you can see just under the cap. This hose is not under pressure, and from the picture (mine is an 85 380), your turns round and exits just in front of a grommet at the left rear of the tank. I'll bet that it droops down to allow coolant escaping after the cap releases to simply dribble on the ground.
I am only 99% sure of what I'm saying here, it's a longer story where the info came from, and I'd welcome correction if wrong. On my filling tank, though, there are only 2 connected hoses under pressure: #5, for filling, and #6, for air returning from the engine and radiator coolant.

Your picture shows a third hose, right on the top of the tank. Where does this go to? It's also not on the diagram (but then, you don't have a sensor on your tank either (#8), so the diagram doesn't match your car or mine.

That said, it appears you've got a broken plastic spigot thing that the hose fastens to. It looks like it just fits into the radiator tank socket? If so, should be a cheap piece.

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1986 560SL
2002 Toyota Camry
1993 Lexus