No VPN access from LAN

[xalib]

I have an OpenVPN server and two OPNsense boxes, which should work as site-to-site clients in this context. The first box works without issues, it holds a stable VPN connection to the server and routes the LAN clients connections to the VPN.

But the second box doesn't, which now left me out of ideas. It has exactly the same configurations of Firewall rules, VPN client (other VPN account though), interfaces... The VPN connection stands and I can ping machines in the VPN from the OPNsense box, but not from its LAN clients. The firewall log shows the ICMP packets from the LAN client to the pinged machine in the VPN as green under the "allow all from LAN" rule. Yet the pong does not arrive at the LAN client.

What could be the issue here?

(I actually remember having had this issue with the first box too, when I set it up. But I didn't document it, which I now regret.)

[xalib]

It is the same in the Interfaces -> Diagnostics tool, I can ping the machine inside the VPN from WAN, but not from LAN.

I went through the easy and straightforward documentation for setting up a Site-to-Site OpenVPN connection a dozen times now, and I understand every step. But I don't understand why the connections coming from LAN aren't routed through the VPN (or at least why there is no response).

[maxxer]

Same/similar here. My office setup: LAN to OpenVPN works. Customer setup: LAN to OpenVPN DOESN'T work