Phaeton does not start

Hello together, my name is Mathias and I’m new here in the forum, I drive a Phaeton TDI 3.0 from 2008. On Friday the problem started. Phaeton started normal and drove to the petrol station to refuel. Car fully fueled. Then wanted to start as always, ignition on, want to start engine, but does not work. The starter does not turn, shift lever is pressed on P, brake pedal is pressed. After 10 minutes try he started at once and runs without error medication, then drove about 20 km. the car starts today again the same, starter doesn’t turn, after about 20 minutes try he jumped on, then drove about 80 km without making out, engine runs without error medication. When starting again the starter doesn’t turn again. I checked the following: – right starter battery has 12.5 volts, – left comfort battery has 11.5 volts, everything measured without ignition – starter ralais in the trunk removed, voltage is given to the coil, closing contact is actuated. . – I noticed that the mass point is corroded on the side of the air filter housing, (above is the filling nozzle of the wiper water tank), but don’t think that it can be due to that? Mass point not yet cleaned, since I have to expand the air filter box. Can someone please give me a tip on what the error can be or has someone already had the same error. Best thanks in advance! Best regards Mathias