Thanks for the reply.
It doesn't seem to be a heat problem. HWMonitor shows almost the same CPU and GPU temperatures for both GPUs:
fanless GTX 750:
- CPU cores at about 39C
- GPU at 37C, fan 0 rpm (30%)
NVIDIA Quadro 2000:
- CPU cores at about 40C
- GPU at 39C, fan 2250 rpm (33%)
If I disable the rear fan, temperatures increase by 1-2C which shouldn't be a problem.
So this indicates that the BIOS has a problem with the missing fan signal. I didn't find a BIOS setting to change this behavior, so my next step will be to try a GTX card with a fan. I'll update this thread with the results.