Net een antwoord op g503.com: "Not sure what type of fuel you have in the Netherlands.
Does it have ethanol in it.
I run into this a lot, here in the states.
The ethanol cleans the metal tank surface and corrosion forms from the natural humidity in the air. Then the corrosion ( the white chalky stuff ) falls into the tank.
Keeping the tank full will keep the air with humidity from getting to the inner tank surface.
This will also help, not stop, the ethanol from absorbing moisture and settling to the bottom, like water.
At first, the moisture laden ethanol settles as droplets, to the bottom of the tank. If the vehicle, your jeep in this case, is not moved, the droplets just lay there and will eventually eat a hole in the metal tank bottom, where they are laying.
Having the tank lined would be another form of prevention."
Lijkt dus met huidige brandstof te maken te hebben. Maar eens goed reinigen en ik overweeg in de toekomst toch een brandstof adidief te gaan gebruiken.