Comment 7 for bug 1299114

I gave up trying to upload that 8.5GB image to U1.

However, I managed to find a probable cause:
http://paste.ubuntu.com/7188828/

As you can see, before trying to add "main" I had 2 networks - "net1" (192.168.111.0/24) and "vlan0" (192.168.50.0/30). The cluster controller has 3 NICs: 1 unmanaged and unconfigured (lo), 1 managed (dhcp+dns) 192.168.50.0/24 (eth0) and another managed (dhcp+dns) 192.168.111.0/24 (eth1). Trying to add a new "main" network with the same IP (network) address 192.168.50.0 fails with the above error. I successfully added a "main1" with the same net address, but a different subnet (255.255.255.252), but failed adding it before changing the name (it said {"name": ["Network with this Name already exists."]}).

So it seems the validation error is correct, but not user-friendly :)