VPN client AND server??

Gents,

I am struggling with setting up my R7000 with Shibby Tomato for months now. I've been able to succesfully configure a VPN client (ExpressVPN) and a VPN Server. They function - individually. However, as soon as I am running the VPN Client, I am no longer able to connect to the VPN Server, externally. If I try to connect while still on the same network, the VPN works. This gives me to conclude that this is a firewall related issue, caused by some setting in the VPN client.

However, the only thing that is firewall related when setting up the VPN client is the 'Firewall -> Automatic' option. Switching this to custom renders the VPN connection to ExpressVPN useless.

Has anyone been able to set up both services simultaneously, succesfully? Pointers, anyone?

Thanks in advance!