17546 / 17544

Audo A6 FSI 4.2 quatro

Recently I am having 17544/17546 error codes, which could among many other things, be a vacum problem.

In most cases i can start my engine without problems when it is cold. But typical after a 1 hour trip, when turning
the engine off and the car stands in the sun, and trying to restart it again after +/- 10/20 minutes, while it is still
hot, it becomes very difficult to start the engine. The starter needs like 10, 15, 20 seconds and only slowly it starts

I did already replace the fuel filter and cleaned the valve house.

I did a quick check with starting fluid.

Can it be something with the AC ? How to check ?