github.com/ValdikSS/openvpn-radiusplugin/issues/14
Openvpn replace "client-cert-not-required" to "verify-client-cert"
Also, verify-client-cert has parameters,
So you need to change it in the Config.cpp
for something like this

// trim leading whitespace
string::size_type  pos = param.find_first_not_of(delims);
if (pos != string::npos) param.erase(0,pos );
pos=param.find_first_of(delims);
if (pos != string::npos) param.erase(pos);
if (param == "verify-client-cert")
{
        this->deletechars(&line);
        if (line == "verify-client-certoptional" || line == "verify-client-certnone")
        {
                this->clientcertnotrequired=true;
        }
}

And recompile