Send a notification email on login attempt
is there a way to get an email notification every time someone tries to login and on successful logon into the OpenVPN-Server?
Not currently, no. That sort of reporting is usually handled by a log or authentication server. If you have the system logs copied to an external syslog server, that may have a way to alert on login since it will cause a predictable entry in the OpenVPN log. If you have an external authentication system (RADIUS, LDAP) then it may have its own reporting.