If you are trying to run jabberd on a Gentoo Linux installation with the portage package, take care to uncomment the <mod_register> and <register> tags in the /etc/jabberd/multiple.xml configuration file.
Or else the registration of new users will certainly be rejected with this error message:
"I wonder", he said to himself, "what's in a book while it's closed. Oh, I know it's full of letters printed on paper, but all the same, something must be happening, because as soon as I open it, there's a whole story with people I don't know yet and all kinds of adventures and battles." Bastian B. Bux