Well, I was going to suggest the fuel tank bladder. Then I saw your location. I don't think the European Gen 2 has a bladder. I've had pump handle click off early lots of times even without a bladder tank, though. But if you put in that much and still have two bars to go, it must have been pretty empty. Unless the fuel gauge is out of calibration, which has been known to happen.
1 US gallon = 3,785411784 liter....so 40 liters = Like others here Ive tried the Fuel Gauge Calibration Instructions on this site about a 100 times and I cant get the sequence to work...