Today I've checked the fuel line, blew the fuel line backwards and this was all open. I decided to refuel at the station 1km away, but on my way it stopped 4 times again. Refilled the tank with 100L and drove back home. In front of the workshop set the engine at 1200rpm and waited 3 minutes untill it died. Then quickly opened the bleeding screw on top of the second fuel filter and it sucked air inside, so there is not enough fuelsupply to the pump. Opened both filters but the fueltap in front of the filters was totally blocked!! Cleaned it all and made a 30min test-drive afterwards. All is working well again including the regulator! A bit strange that I could start the engine so quickly after it ran dry, but it is obvious that the USA build machinery is though to break!
Thank you all for your helping and the replies! Tomorrow the tractor will go to a local show to shine!