No traffic between bridged network and other networks

[racer]

Hi!
I have an issue with bridged networks and hope you can assist me in finding my mistake!
Background: I have a small server with OPNsense installed as a virtual machine (KVM) on top of Ubuntu 20.04. There are 2 physical onboard NICs, one USB NIC, a virtual host-only network and a WiFi interface. Aside from the WiFi interface the other interfaces are connected directly to the VM.
Additionally I created several VLANs for one of the onboard NICs. So far, so good. Everything (DHCP, DNS, routing, Wireguard VPN,...) is running smoothly.

Now I needed to create a bridged network because I wanted one VLAN and the USB NIC to be on the same network. So I did the following:
- removed static IPs from both the VLAN and the NIC network
- created a bridge and added the NIC and the VLAN
- assigned the bridge to a new network
- configured the bridged network with static IPs
- changed the tunable settings so filtering is done on the bridge interface
- created a new firewall rule on the bridge network: allow any->any (at first, I created other rules but for troubleshooting purposes I defined the new rule)
- enabled DNS and DHCP for the interface

I found several manuals that are quite similar and include those steps.
My issue is: there is no traffic between the clients connected to one of the member networks and my other networks. DHCP is working (clients get an IP, DNS, gateway assigned), but no other connection is possible. I cannot even connect to the the OPNsense web interface. But I also see no blocked traffic on the firewall (Live view).
I already deleted the whole setup and recreated it from scratch with the same outcome.
Any ideas? 

[racer]

I now disabled the whole firewall... still no traffic coming through. Could it be a routing issue?