Newest kernel breaks somehow my networking. I managed to get it working by changing kernel at boot.
Working fine:
Linux proxmox 5.4.65-1-pve #1 SMP PVE 5.4.65-1 (Mon, 21 Sep 2020 15:40:22 +0200) x86_64
root@proxmox:~# systemctl status networking
● networking.service - Network initialization...