Networking start failed on OVH Proxmox 6.4 if a bridge has no interface
The problem comes from /etc/network/if-pre-up.d/wait_for_link_up, which will wait forever on vmbr1. It looks like this script was recently added by OVH on their Proxmox template, as it does not exist on our older hosts...
Anyway, for now, I fixed the problem by excluding the vmbr1 interface with:
if [ "x${IFACE}" = "xvmbr1" ]; then
exit
fi