I deleted the chatroom and added it again, based on the link on the miranda-ng website. The thing auto-populated from my clipboard.
<presence to="xmpp:miranda-ng-int@conference.jabber.ru?join/ultramage">
<priority>
0
</priority>
<x xmlns="http://jabber.org/protocol/muc" />
<c xmlns="http://jabber.org/protocol/caps" hash="sha-1" node="http://miranda-ng.org/caps" ver="pIkQaCwAC+LHTyKhh82FVQnGgo0=" ext="mirotr x64 mood activity mir_notes" />
<x xmlns="vcard-temp:x:update">
<photo />
</x>
<x xmlns="miranda:x:vcard">
798d9742b77d9971d7a2daadab48b0e3
</x>
<status>
</status>
</presence>
<presence from="xmpp:miranda-ng-int@conference.jabber.ru?join/ultramage" to="********@jabber.org/Miranda" type="error">
<error code="400" type="modify">
<jid-malformed xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
</error>
</presence>
If I understand right, the jabber.org server talks to jabber.ru on my behalf, so it might be modifying/garbling my message on the way. I wouldn't be able to tell. It's been fine for a little while after I revived my old account, I used it to join the chatroom and I talked to you for a bit about my initial problems with the Discord plugin and how I was not able to reach cdn.discordapp.com from my current location. Afterwards, it broke and I was unable to properly re-join the chatroom. I thought it was some recent code change in Miranda-NG that broke something.