Router swap, test device, wireguard not receiving

[ghwqer]

Hi,
i want to replace my legacy router with an OPNsense DEC750 router. Before switching everything over, I wanted to test whether everything works correctly and install the firmware on a Supermicro mainboard with an 8th gen i5-U CPU.

I set up the following test environment:
Local network (172.x.x.x)
-> OPNsense router
192.168.1.x network

I configured everything, and during the first tests, everything worked fine. Now I need to set up WireGuard. I followed both the Thomas-Krenn guide and the official OPNsense documentation.

On the WAN side, I disabled:
Block private networks
Block bogon networks

In the OPNsense WireGuard status page, the peer is sending and receiving data, but the icon stays red. On the WireGuard client, the received bytes always remain at 0.

I have already tried a lot of things:
Allowed all IPs on the client side
Regenerated and changed the keys
Imported the configuration directly from the router

The error is always the same.

Does anyone have some hints for troubleshooting? I already tried debugging with ChatGPT and Gemini, but none of the suggestions solved the issue. Could this maybe be a hardware issue?

[ghwqer]

The client log tells me some Handshake error every 5 seconds