The fix to the problem is to change /etc/NetworkManager/nm-system-settings.conf from:
[main] plugins=ifupdown,keyfile [ifupdown] managed=false
[main] plugins=ifupdown,keyfile [ifupdown] managed=true
and you're good to go.
In my case the problem started after installing the vlan package.
howtos/networkmanager_and_device_not_managed.txt · Last modified: d/m/Y H:i by 127.0.0.1