iGPU Passthrough: Machine won't start after adding PCIe device

[BerndDasBrot]

I have a ASRock N100m mainboard/cpu, that should support iGPU passthrough just fine. To enable this functionality, I followed these steps:

1. Update GRUB config to add Kernel parameters
2. Adding VFIO modules
3. Blacklist i915

(As described here: https://peanut-tech.com/how-to-pass-through-intel-igpu-to-proxmox-virtual-machine/)

After rebooting Proxmox, I can now add the PCIe device to my VM (Raw device with proper address, all function, ROM-Bar, PCI Express all enabled).
I also changed the machine type to "q35".


But when I start the VM (Ubuntu Server), it gets stuck during startup:

When I remove the PCIe device from the machine again, it starts up just fine.
Does anyone have a hint how to make this work?