I saw a similar comment on the pve user mail list.
the email subject is: Network config (VLANs) changed in 4.0?
here are some parts:
"i reinstalled proxmox 4.0 and took over the /etc/network/interface file from my 3.4 installation.
So far it is running well.
But if i want to change a network setting via the network GUI I get an error message:
bridge 'vmbr41' - unable to find port 'eth0.41' (500)
A test adding the lines "auto eth0.41" and "auto eth0.77" solves the problem but is not necessary for debian itself."
and
"because the management interface is part of a vlan also.
For the records - i changed my interfaces file to this:
auto lo
iface lo inet loopback
auto eth0
# Management interface (Proxmox Web GUI)
auto eth0.41
iface eth0.41 inet static
address xxxx.xxx.1.164
netmask 255.255.255.0
# base for the VMs
auto vmbr0
iface vmbr0 inet static
address 0.0.0.0
bridge_ports eth0
bridge_stp off
bridge_fd 0
Omitting the address from vmbr0 does not bring it up.
Thanks for the hints. They put me into the right direction."