Full error message from creation page :
.. failed to start : lxc-start: confile.c: config_network_ipv4: 835 File exists - invalid ipv4 address: / lxc-start: parse.c: lxc_file_for_each_line: 57 Failed to parse config: lxc.network.ipv4 = / lxc-start: tools/lxc_start.c: main: 284 Failed to create lxc_container
And actually the network line looks like : lxc.network.ipv4 = /
Which is of course wrong ! Fixing it manually solves the issue and container starts properly.
The network was set to automatically assign an adress in creation page, this system runs others KVM and network is ok, IP ranges are assigned.