Vaillant EcoTec 638 reaching max pressure 3.1 and cutting out

Hiya - Something has happened to my boiler, 2 year old, service March 2020, the boiler is packing up 15mins into the heating run in that it is cutting out the flame and waiting for pressure to drop back down from max 3.1. It is then stuck in the loop. There is no air in my radiators, also no air in the magna clean. I have 3 by-pass valves, 1) radiator circuit 2) UFH circuit 3) hot water. The rads heat up fine suggesting valve is opening. So far, my hot water and UFH circuits are not affected, it is only the radiator circuit that is showing this problem... Is this air inside boiler mechanics or expanssion vassel ?