@Draco By any chance you upgraded the pfsense (and or openvpn package) recently ? I got 'similar issue' that left me baffled till this day see here , maybe it is similar with what you experiencing.

@adminproconer said in Firewall rule problems. (Client-to-client forward): Where should I start troubleshooting the issue? With the network settings and firewall config of the concerned device. Ensure that all devices in both subnets use pfSense as gateway. If you can access a device from within it's own subnet, but not from another network segment check its firewall and ensure that it allows access from outside.