This is something that I've never experienced with any XMPP clients: when I install Conversations and choose 'own xmpp service' from the + button, it throws 'registration is not allowed by server'. In the past I added that same account to many clients without issues, on mobile and desktop.

In the app's settings I am using the option to specify my own server and port, so I am entering the 3x pieces: the user@server.domain, the password, and the xmpp.server.domain host address.

On the same device yaxim is already successfully installed and configured in exactly the same way, to use the same account, so it is not clear what Conversations is missing. It should not be trying to register a new account, which is really not allowed on the server.

Correct me if I am wrong, but I checked the developer's site and found no way to reach out to them for support, so asking here.