UR44C No signal on DAW input only for Mix 2 (FW 3.20)

Hello, I recently acquired a UR44C interface and now I’m facing a problem:

When sending audio from Windows to the WASAPI “DAW input” (PC return 1) no signal is present in Mix 2 - and only this input only in Mix 2.
Mix 2 can receive the “Music” (PC return 2) and “Voice” (PC return 3) inputs just fine.
All other mixes receive audio as usual on all three stereo returns from the computer.

Maybe this is a possible bug in the firmware / DSP program.

Model: UR44C
Firmware: Main 3.20
Platform: Windows 11

Cheers from Germany, Lars