What OS are you using in the front-end? Do you have SELinux or PolKit or AppArmor enabled?
AppArmor: (Yes)
root@opennebula-01:~# cat /sys/module/apparmor/parameters/enabled
Y
But SELinux and PolKit i don;t know how to check in Ubuntu 16.04. It is a clean install.
Can you try disabling AppArmor?