Could you try for at small test only assigning 1 socket to the VM - 8 cores in total?
When I lost hope for positie solution, i decided to make several VMs with smallest performance. For tests to current (problem) VM i attached 8 cores 1 socet, and ..... BINGO!
But question is open, what mean socet in proxmox VM configuration, is it equal to thread on host server, and VM with 7cores 2 socet was invalid configuration or Bug (proxmox or windows)?