The fix to the problem is to change /etc/NetworkManager/nm-system-settings.conf from:

<file>
[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=false
</file>


to:
<file>
[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=true
</file>

Then run:

<code>
/etc/init.d/network-manager restart
</code>

and you're good to go.

In my case the problem started after installing the vlan package.