Update: I was NOT COMPLETELY WRONG... the "freezing" (so to say - the spinner turns infinitely) is gone... but the performance is still a lil bit poor. But over the night, Windows 10 has not given up working.
Please try again with latest package version. If your server cannot run 5.13 kernel, try our 5.15 kernel (apt install pve-kernel-5.15) and make sure that you got latest qemu installed (pve-qemu-kvm: 6.1.0-3)