I've been trying to set up a connection with the VPN service build in in MacOS between two Macs in my local LAN. Both are connected with the clearOS router on the LAN side but at different interfaces/networks. (It works when they are in the same network.)

The last meaningful step that I can see in the logs on the server is that it receives the L2TP SCCRQ and sends the L2TP SCCRP. It looks like the client is never receiving the SCCRP.
Does anybody know whether clearOS has difficulties in forwarding L2TP requests and responses back and forth? Do I have to configure any firewall settings or port forwarding or stuff like this even if both computers are on the LAN side of the clearOS router?