Check that OpenVPN tunnel network does not overlap any other subnet
Lots of newbies just paste 10.0.8.0/24 as the tunnel network on their OpenVPN instances. This comes from an example setup somewhere. Then they put the same tunnel network in multiple OpenVPN instances (servers and clients). The tunnel networks should all be different, and different from any subnet defined on any interface.
In the GUI validation of OpenVPN settings, check that the tunnel network does not overlap any other tunnel network, or any other subnet. Check needed for both IPv4 and IPv6 tunnel network boxes.
Are there any unusual use cases where the same tunnel network is OK on multiple OpenVPN instances?