This happens when user tries to create a new network with a duplicate vlan ID:
vagrant@ubuntu:~/devstack$ nova network-list
+--------------------------------------+---------+-------------+
| ID | Label | Cidr |
+--------------------------------------+---------+-------------+
| a16f1896-da4c-4978-9776-0b6368129b07 | private | 10.0.0.0/24 |
+--------------------------------------+---------+-------------+
vagrant@ubuntu:~/devstack$ nova network-show a16f1896-da4c-4978-9776-0b6368129b07
+---------------------+--------------------------------------+
| Property | Value
+---------------------+--------------------------------------+
| bridge | br100
| bridge_interface | eth1
| broadcast | 10.0.0.255
| cidr | 10.0.0.0/24
| cidr_v6 | -
| created_at | 2015-02-10T05:52:41.000000
| deleted | False
| deleted_at | -
| dhcp_server | 10.0.0.1
| dhcp_start | 10.0.0.3
| dns1 | 8.8.4.4
| dns2 | -
| enable_dhcp | True
| gateway | 10.0.0.1
| gateway_v6 | -
| host | -
| id | a16f1896-da4c-4978-9776-0b6368129b07
| injected | False
| label | private
| mtu | -
| multi_host | False
| netmask | 255.255.255.0
| netmask_v6 | -
| priority | -
| project_id | -
| rxtx_base | -
| share_address | False
| updated_at | -
| vlan | 100
| vpn_private_address | 10.0.0.2
| vpn_public_address | -
| vpn_public_port | 1000
+---------------------+--------------------------------------+
vagrant@ubuntu:~/devstack$ nova network-create --fixed-range-v4 20.0.0.0/24 --vlan 100 vlan100
ERROR (Conflict): There was a conflict when trying to complete your request. (HTTP 409) (Request-ID: req-634ac773-3fc7-4013-a319-69fbd4f1cca3)
Contrary to other conflict such as subnet conflict, nova returns a clearer error message.
@Kahou, can you see errors / stack traces in any of the nova logs? can you please upload them to this bug? Which version of Nova? Are you using Nova Network? (and no Neutron just to confirm).