Fuel filler neck

That sounds more like the tank breather system is blocked. It connects to the tank and the vapor is collected in the EVAP canister until the purge solenoid opens and pulls the vapors out of the canister. If the vent valve sticks closed there is no way for the vapor in the tank to move as the tank fills with fuel and it "burps" up through the filler neck. In the case of your car that vent is part of the Leak Detection Pump assembly that is inside the rear wheelhouse behind the tire under the filler neck The control valve for the vent is stuck or the filter assembly is plugged, it will look similar the attached images. The replacement part will depend on which filler neck you have, but the procedure to replace it is to access it behind the trim cover, then disconnect the hose to the EVAP can, remove the electrical connector and loosen the bottom bolt in the backing plate and slide it down and off to remove it. Reverse the process to install the replacement. You could get the filter and the valve as replacement parts but now the complete assembly is usually lower cost.

thank you very much, that helps me out significantly.

You're welcome, please let us know what you find. A simple way to verify that part is the issue would be to get to it and just pull the hose free on the tank side when you have a low tank of fuel. Then try filling it. If you can add fuel with no issue then reconnect it and try again. The line should be a quick disconnect that you squeeze to remove it. Would show you quickly if it was the issue or not and not cost you money.