OpenVPN configuration on Buffalo WZR-HP-G300NH

I have an old Buffalo WZR-HP-G300NH router which I am trying to config to use FastestVPN with the help of the manual - I have a fresh install of OpenWrt 19.07.1. When I am getting to the part when I need to add new network interface - I get an error saving the configuration. If I do not revert back, router became unavailable and I need to do a hard reset.
Any advise what can be wrong?