Troubleshooting KARL (no signal at OUT1)

Hi! I just got a KARL PCB with SMD presoldered. After assambling the OUT1 there's no signal at OUT1.

Signal in IN1 (only) => output at MIX and OUT2 (but not OUT1).

Signal in IN2 (only) => output at MIX and OUT2.

Signals in IN1 and IN2 => output signal from IN2 at MIX and OUT2 (but nothing at OUT1).

It's not a problem with the OUT-Jack, same problem while checking signal at the resistor in front of the OUT1-Jack.

Does anyone know the problem with this pre-soldered module?

Is it perhaps possible to get the schematics to continue tracing?

Thanks!