Ad-hoc silently failing

When doing /ad-hoc webhooks.cheogram.com, pressing enter on "new" action produces nothing in UI. In XMLTab, i see that that poezio sends an error to the component:

<iq to="webhooks.cheogram.com" type="error" id="bd6bf8fd21e8477fb0ec3d1945a2ff14"><error type="cancel"><undefined-condition xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" /><text xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">Slixmpp got into trouble.</text></error></iq>

To reproduce, you can use that public instance, which requires in-band registration:

/rawxml <iq type='get' id='reg1' to='webhooks.cheogram.com'><query xmlns='jabber:iq:register'/></iq>
/rawxml <iq type='set' id='reg2' to="webhooks.cheogram.com"><query xmlns='jabber:iq:register'><username>USERNAME</username></query></iq>