Hi,
I had exactly the same problem as you all have reported.
I ordered a Brake servo pressure sensor from the Audi Parts Department at my local Audi dealership.
It was really really easy to replace like for like. I have no experience with car maintenance. I’ve got the A4 and it was to the top left of the engine compartment, under a plastic covering that lifted up. I’d recommend having a go. It cost me £75 for the part.