I was trying to debug an OpenMediaVault - OpenVPN (4.0.4) plugin issue and discovered that the server certificate was overritten with a client certificate under /etc/openvpn/pki/issued/<hostname>.crt
To reproduce, create a new client with a Common name set to the server hostname. This will overwrite the server certificate with the new client certificate and break all VPN connections.
Could a little error checking be added to the web interface to block this?
To fix, I had to uninstall/re-install the openvpn plugin and re-create all of the users.