I solved the Problem:
Shutting down the unraid server and edit the USB-Stick in a windows machine.
Removed the file /config/network-rules.cfg and edit the file /config/network.cfg to default values:
# Generated network settings
USE_DHCP="yes"
IPADDR=
NETMASK=
GATEWAY=
BONDING="yes"
BRIDGING="yes"
Now the routing of virbr0 is working as it should and noVNC is working as well.