systemd networking.service fails after last dist-upgrade

[leesteken at pm.me]

Dear Proxmox user list,

My network and internet connections work fine but I'm getting a "State: degraded" from systemctl status,
since an apt-get dist-upgrade yesterday. That Proxmox (6.1-7) update did rewrite my /etc/network/interface
configuration file, but it looked harmless:

#cat /etc/network/interfaces
auto lo
iface lo inet loopback

iface enp4s0 inet manual

auto vmbr0
iface vmbr0 inet static
        address 172.17.2.6/24
        gateway 172.17.2.1
        bridge-ports enp4s0
        bridge-stp off
        bridge-fd 0
        hwaddress 52:54:56:17:02:06

However, today after a reboot, I'm getting the following errors:

#systemctl status networking.service
● networking.service - Network initialization
   Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2020-03-15 09:15:50 CET; 25s ago
     Docs: man:interfaces(5)
           man:ifup(8)
           man:ifdown(8)
  Process: 13819 ExecStart=/sbin/ifup -a (code=exited, status=0/SUCCESS)
  Process: 13894 ExecStart=/sbin/ifup --allow=ovs (code=exited, status=1/FAILURE)
 Main PID: 13894 (code=exited, status=1/FAILURE)

#systemctl restart networking.service
mrt 15 09:15:50 sentry systemd[1]: Starting Network initialization...
mrt 15 09:15:50 sentry systemd[1]: Reloading Postfix Mail Transport Agent (instance -).
mrt 15 09:15:50 sentry postfix/postfix-script[13854]: refreshing the Postfix mail system
mrt 15 09:15:50 sentry postfix/master[4874]: reload -- version 3.4.8, configuration /etc/postfix
mrt 15 09:15:50 sentry systemd[1]: Reloaded Postfix Mail Transport Agent (instance -).
mrt 15 09:15:50 sentry systemd[1]: Reloading Postfix Mail Transport Agent.
mrt 15 09:15:50 sentry systemd[1]: Reloaded Postfix Mail Transport Agent.
mrt 15 09:15:50 sentry kernel: vmbr0: the hash_elasticity option has been deprecated and is always 16
mrt 15 09:15:50 sentry systemd[1]: Reloading Postfix Mail Transport Agent (instance -).
mrt 15 09:15:50 sentry postfix/postfix-script[13885]: refreshing the Postfix mail system
mrt 15 09:15:50 sentry postfix/master[4874]: reload -- version 3.4.8, configuration /etc/postfix
mrt 15 09:15:50 sentry systemd[1]: Reloaded Postfix Mail Transport Agent (instance -).
mrt 15 09:15:50 sentry systemd[1]: Reloading Postfix Mail Transport Agent.
mrt 15 09:15:50 sentry systemd[1]: Reloaded Postfix Mail Transport Agent.
mrt 15 09:15:50 sentry networking[13894]: error: main exception: no ifaces found matching given allow lists
mrt 15 09:15:50 sentry systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
mrt 15 09:15:50 sentry systemd[1]: networking.service: Failed with result 'exit-code'.
mrt 15 09:15:50 sentry systemd[1]: Failed to start Network initialization.

Can anyone see what I have done wrong? Has someone experienced the same issue?
Is there anything I should check or correct? (I have IPv6 disabled in sysctl because I don't use it.)

kind regards, Arjen