Referring to @pagaille experience…
https://community.nethserver.org/t/how-do-i-give-access-to-all-lan-machines-to-a-openvpn-tunnel-route-is-not-created-when-using-a-subnet-other-than-24/
If a subnet is wrong into the form the route add fails.
Maybe is not possible to check/validate every subnet, but at least:
docs could mention that data has to be verified or double checked routes
or
if route add fails this should be forwarded into GUI somehow.