If you’re also running into trouble with openvswitch-switch and really have no clue why it is not running as it should, you might also have a mismatch of a default no-subscription installation (it comes out of the box this way) and the default repo configured to work with the subscription/enterprise version only.

Unless you have a subscription key you will never get any updates for the PVE system itself – so before changing the network configuration to work with openvswitch-switch, change your repo in /etc/apt/sources.list.d/pve-enterprise.list as follows:

#deb https://enterprise.proxmox.com/debian/pve stretch pve-enterprise
deb http://download.proxmox.com/debian/pve stretch pve-no-subscription

Update the package lists (apt update) and upgrade (apt upgrade) – you will get updates for openvswitch-switch!