Activate SSH in DSM, login through SSH with admin account.
Then enter following:
echo log /var/log/openvpn.log > /usr/syno/etc/packages/VPNCenter/openvpn/openvpn.conf
echo verb 4 > /usr/syno/etc/packages/VPNCenter/openvpn/openvpn.conf
sh /var/packages/VPNCenter/target/scripts/openvpn.sh restart
To read the log:
cat /var/log/openvpn.log