Made some progress with more careful examination of the systems where it works and where it doesn't, and it turns out that there is also an openvpn3-client package that has to be installed as well as openvpn3. This didn't occur to me because the openvpn3 client binary was installed. But when I installed openvpn3-client, the missing binary for the session manager was installed. So now I can get a little farther; I just need to figure out why user authentication is failing (I've tried it on the old system so I can verify I have the correct user name and password, and I have tried it enough times on the new system to rule out simple password typo as the cause). So now I can execute the session-start command, and it does prompt me for the username  and password, but always returns authentication failure.

--Greg