"When i looked to the wall plug, i saw that it's the old, 3-holes type, while the connector of the extension cable has only 2 pins plus the earth which is not a 3rd pin, but a clip in the sides of the connector."
thats are "schuko" type plugs.
The 2 metal clips are connected to the earth. Anyway check with a mutimeter (continuity check) if that clips AND the center hole of the 3 holes plug, are phisically connected.
