If you can connect using other clients, then this is very likely an issue with how you use the library, not in Openfire.
Every XMPP server has a domain, even if you do not configure a public “nice” one. It is important to define that correctly when setting things up. Look at the first page of the admin console of Openfire: it will show you what the domain name is that was used to configure Openfire. You need to use this domain value when connecting with a client.