With the newest version v3.2.0-beta.2 as far as I tested, everything works fine now 
. Thanks to @sualko for the update!
As on apps.nextcloud.com it is listed as “unstable”, it is so far not possible do update it via app store from out of nextcloud web ui. But you can download the archive and extract the tar.gz and contained tar archive manually to get the ojsxc folder for nextcloud/apps.
€: Okay, while using the internal xmpp server and therefore leaving all admin panel settings empty, I still get some errors:
Error PHP Undefined index: timeLimitedToken at /var/www/owncloud/apps/ojsxc/ajax/setAdminSettings.php#34 2017-05-01T18:35:37+0200 Error PHP Undefined index: xmppPreferMail at /var/www/owncloud/apps/ojsxc/ajax/setAdminSettings.php#26 2017-05-01T18:35:37+0200 Error PHP Undefined index: xmppStartMinimized at /var/www/owncloud/apps/ojsxc/ajax/setAdminSettings.php#25 2017-05-01T18:35:37+0200 Error PHP Undefined index: xmppOverwrite at /var/www/owncloud/apps/ojsxc/ajax/setAdminSettings.php#24 2017-05-01T18:35:37+0200 Error PHP preg_match() expects parameter 2 to be string, array given at /var/www/owncloud/apps/ojsxc/ajax/setAdminSettings.php#7 2017-05-01T18:35:37+0200 Error PHP Use of undefined constant script - assumed 'script' at /var/www/owncloud/apps/ojsxc/templates/settings/admin.php#5 2017-05-01T18:35:22+0200
But chatting nevertheless works, as far as I could test.