Automatic transmission memory error

I have a V6 TDI engine with code KB BAU and an EYJ Tiptronic transmission installed in my VW Bus T3. The following error is displayed in the transmission's fault memory:

18265 Last signal error message from the engine control unit: P1857 -35-00--.

The engine isn't responding to the accelerator, and the transmission is shifting roughly. I haven't installed the ABS control unit because it doesn't fit.
What could be the reason for that? Thank you in advance.

Best regards, Kashi.

"I had similar issues (also known as a load signal problem) with my ESP or ABS system (I'm not sure which one), and it turned out to be the connector for the mass airflow sensor (MAF) that wasn't properly connected. Take a look at the MAF sensor readings."