There is no blockage in the expansion pipe. What can the problem be, and how do i solve it?

The pipe i think you are describing is an open vent the tank is a feed and expansion tank it sounds like you are saying expansion is going up into the feed and expansion tank. What is the problem????????? look-water is supposed to raise into the feed and expansion tank by way of the cold feed pipe(this is the water expanding in the system thus raising the water level)water would only enter the tank by way of the open vent if the pipework had been badly run and allowed the pump to over run through the open vent which would cause another set of problems we do not need to go into seeing as you do not have that problem. The only thing that we will all be wondering is why have you replaced the thermostat, three port valve and pump because so far you have not actually described any sort of problem.

If this does not overflow to outside what you describe is thermal expansion. Only air should ever go up the expansion ( vent ) pipe.