***Note: As of this writing, if you’re using x2go client there is an issue with network-manager an D-bus.
This can occur if network-manager and network-manager-openvpn were installed after the system was setup.
Try editing /etc/NetworkManager/NetworkManager.conf and change managed=false to managed=true as below, then reboot.
[main]
plugins=ifupdown,keyfile
[ifupdown]
managed=true
This can occur with Ubuntu 11, 12, 13, 14 etc…
http://ubuntuforums.org/showthread.php?t=1847900&p=11273302#post11273302