route -q -n add -inet 192.168.1.0/24 -interface wg0The line above indicates that this should be a network on the other side of the VPN tunnel.If one of your local interfaces has this network, wireguard will break. In 20.1 it seems FreeBSD just ignored this.
WHERE did you set this 192.168.1.0/24? in local instance or endpoint?
I checked all available good documentations and also the official ones:https://www.routerperformance.net/opnsense/opnsense-and-wireguard/I have no idea why you set your local networks in local instance. This is nowhere documented. Maybe this would was dismissed with FreeBSD 11.2 and now throws an error in FreeBSD 12.1