Issue with OpenVPN inside a LXC container: Failed at step NAMESPACE spawning /usr/sbin/openvpn: Permission denied

[Simon Bernier St-Pierre <sbernierstpierre@gmail.com>]

I have a LXC container which is connected to a remote VPN using OpenVPN. 
After upgrading to buster, the VPN does not start anymore.

I'm using Debian buster on my host OS and Debian buster on the guest OS. 
Both were updated from stretch. Aside from OpenVPN there's only deluged 
and deluge-web that are installed.

This is the journalctl log for the openvpn service:

root@dl:/# journalctl -u openvpn-client@pia.service
-- Logs begin at Tue 2019-07-16 20:32:30 EDT, end at Tue 2019-07-16 
22:31:19 EDT. --
Jul 16 20:32:30 dl systemd[1]: Starting OpenVPN tunnel for pia...
Jul 16 20:32:30 dl systemd[70]: openvpn-client@pia.service: Failed to 
set up mount namespacing: Permission denied
Jul 16 20:32:30 dl systemd[70]: openvpn-client@pia.service: Failed at 
step NAMESPACE spawning /usr/sbin/openvpn: Permission denied
Jul 16 20:32:30 dl systemd[1]: openvpn-client@pia.service: Main process 
exited, code=exited, status=226/NAMESPACE
Jul 16 20:32:30 dl systemd[1]: openvpn-client@pia.service: Failed with 
result 'exit-code'.
Jul 16 20:32:30 dl systemd[1]: Failed to start OpenVPN tunnel for pia.

I've searched online but I haven't found anything relevant to my situation.

Anything to help me figure this one out will be greatly appreciated. Thanks!