En la configuración de jabberd, en la parte donde esta la configuracion de registro, me parece que tienes el siguiente comentario:

If you want to allow inband-registration, you have to
request at least one element (e.g. just the
element). If you remove all requested elements, jabberd14
will not allow inband registration, but will still reply
to registration get requests. (I.e. you can then use the
content of the element to give additional
information on why registration is not possible, or you can
use it to redirect the client to a web page, where account
registration is possible.


Así que ahí dice lo que tienes que hacer.