never manage it myself. like OP , the clone would not boot. at all.
manage to clone win7 though, with a little wrinkle here and there but it did worked.
although I did that on Hyper-V setup not Proxmox.
well initially I just made a copy of the virtual drive file and attached it to new VM, that did not work.
so after that I boot up the VM and run sysprep.
shut down, Export the VM and than re-import the VM as new copy
but remember it was MS Hyper-V not proxmox. I do not run windows on proxmox as yet. maybe when I get my home server up and running I might try that but I have very little use for windows in VM at home.