When starting in webui, the error is not explicit. In console :
root@proxmox:~# qm start 104no physical interface on bridge 'vmbr3'
kvm: -netdev type=tap,id=net0,ifname=tap104i0,script=/var/lib/qemu-server/pve-bridge,downscript=/var/lib/qemu-server/pve-bridgedown: network script...