brandtwein wroteThe HOWTO only mentions that by default OpenVPN requires both client-cert AND username/password
Where does it say that .. please quote it.
The recommended way to authenticate is with a full PKI - ie: via certs
not username/password.
I do not think it is possible to have one server configured to allow some users authenticate via certs and some others via username/password. For that you would need two server configs .. each setup as required.