No traffic on WAN interface possible

[sguenther]

Hello,
I'm running version 21.7.1 on a proxmox server with

LAN: em0 10.0.0.1/24
WAN em2 192.168.178.9/24

All firewall rules on the proxmox server are deactivated, this has been successfully tested with other vms.

The gateway is a Fritzbox (192.168.178.1).
If I configure the OPNsense as gateway (masquerading is configured), the clients have no internet access.
When I ping the Fritzbox, tcpdump tells me that there is a connection.

09:44:07.564549 IP (tos 0x0, ttl 64, id 34149, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.178.9 > 192.168.178.1: ICMP echo request, id 64897, seq 1, length 64
09:44:07.564906 IP (tos 0x0, ttl 64, id 62457, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.178.1 > 192.168.178.9: ICMP echo reply, id 64897, seq 1, length 64

But the ping command doesn't return anything.
I have alreday deactivated the blocking of private and bogon networks, but that didn't help.
For testing purpose the WAN interface is currently accepting any traffic. but still not feedback from ping and no internet access.

I'm stuck and I'm sure that only a detail is missing.

Thanks for any help or suggestions,

Stefan