Audi A4: Electronic Problems & Solutions

Good morning!
I'm having a problem with an Audi A4 3.0 TDI Quattro Tiptronic Avant, manufactured in 2005.
MKB:BKN GKB: CGX

The car runs normally when cold, and everything functions as it should. However, after driving for 10-20 kilometers, the...
The gear indicator in the instrument cluster switches to manual Tiptronic mode, and it downshifts all the way to 1st gear! You can then shift up, but it won't...
It was immediately switched back on. The selector lever remained in the "D" position the entire time.

The transmission's fault memory contains the following error!
Switch for Tiptronic (F189) down: Short circuit to ground.

The following items have already been exchanged and checked!
2 new circuit boards in the selector lever.
Mechatronics system complete with control unit and wiring harness, installed in the transmission!!
Fahrzeug wurde auf Wassereinbruch geprüft. Everything's fine!

Who can help with this problem?!