'ValidationError' object has no attribute 'error_dict' when creating a network

Dimiter, I can't reproduce this. Can you check that all dependencies on the machine are up-to-date?

I did apt-get update & upgrade and still get the same issue. Did some search for this issue in django (v1.6.1) bugs, but nothing definite. What info will be useful? I suppose I can try uploading the machine and its disk image to U1 so you can try it, along with the setup that's needed..

Uploading the disk image sounds like a pain, but it would be massively useful.

I'm preparing the upload and will post links as it's done.

I took me 3h just to tar.bz2 the disk image (21GB down to around 10GB).

Can't reproduced this either. Using a dev instance, I ran the command Dimiter gave me and it didn't crash:
$ ./bin/maas maas networks create name=main ip=192.168.50.0 netmask=255.255.255.0 description="Main MAAS network"
{
    "description": "Main MAAS network",
    "ip": "192.168.50.0",
    "netmask": "255.255.255.0",
    "vlan_tag": null,
    "resource_uri": "/api/1.0/networks/main/",
    "name": "main"
}

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 :)

In theory we do not allow multiple networks with overlapping (i.e. identical or nested) address ranges. In the model code, Network.validate_unique checks against such clashes, and raises a ValidationError. Maybe this needs to be a different, more specific error; or maybe the form code needs to catch the ValidationError and attach the error to a specific field.

Got it. This doesn't become clear in the documentation as far as I can see, and even runs counter to the examples, but when validate_unique raises a ValidationError, it has to be one that was constructed from a messages dict! If not, kaboom inside Django.