problems with the trailer lighting

I recently bought a second-hand Skoda Octavia IV. Now I have a problem with the lights of the trailer. The turn signals work correctly, but when I brake, the turn signals are also on instead of the brake lights.

This is almost certainly something that has been adjusted in the programming, since the lights flash when you activate both the brake light and the turn signal, which I think rules out a wiring error (which is also confirmed to me in my garage).

Does anyone know which parameter(s) should be adjusted? I have an obdeleven to do this myself if necessary.

For information: this problem is not covered by the warranty insurance (checked) and the garage simply wants to replace the entire control unit at 1000 euros

Thanks in advance