Compiling a kernel (with hopefully working fix atm). I had iommu=pt in my grub parameters and the issue persisted, however I do not remember if I ever checked the logs with the controller not passed through. I will update this once the server is done compiling.
Here the update: As suspected...
SYSTEM:
My system is an HPE ML350 G6 using a Xeon 5645 running ProxmoxVE 6.3-3. VT-d enabled in BIOS and IOMMU enabled in GRUB settings.
As my system is affected by the rmrr-issues, my kernel has the fix patched in...