@steff22 I have the exact same problem with a Asrock rack B650D4U i tried everything to make it but work but have the exact same problem as you.
Even new fresh installation of windows machine didnt work.

If you have solution please let me know thanks in advance!

@Houbsi yeah i did try it with the nvcleaninstall but gives me the same error and also tried the HV value to hide windows is virtualised.

Will try again to remove everything and start over

Is it known GPU passthrough dont work in 8.3 RC1 ?
i did the setup now through xen-orchestra and see it in windows but no matter what i do it keeps giving error code 43