Bugfix.

https://xmpp.org/extensions/xep-0198.html#resumption

The <resume/> element MUST include a 'previd' attribute whose value
is the SM-ID of the former stream and MUST include an 'h' attribute
that identifies the sequence number of the last handled stanza sent
over the former stream from the server to the client.

src/headless/converse-smacks.js

@@ -159,7 +159,7 @@ converse.plugins.add('converse-smacks', {
             _converse.connection._addSysHandler(_.flow(onFailedStanza, promise.resolve), Strophe.NS.SM, 'failed');
 
             const previous_id = _converse.session.get('smacks_stream_id');
-            const h = _converse.session.get('num_stanzas_handled_by_server');
+            const h = _converse.session.get('num_stanzas_handled');
             const stanza = u.toStanza(`<resume xmlns="${Strophe.NS.SM}" h="${h}" previd="${previous_id}"/>`);
             _converse.api.send(stanza);
             _converse.connection.flush();