For the users who are searching for an alternative secure XMPP server: I have installed prosody now as debian package. https://prosody.im/
The configuration is well documented and very easy in comparison to jabberd. I can use a self-signed certificate or let's encrypt without problem. All the communication and file transfer is encrypted with ECDHE-RSA-AES256-GCM-SHA384. I have verified this in an wireshark session. It's simply perfect.
