I found an entry for that error in Kibana:
extension src id undefined points to non-existent element for: <span typeof="mw:Extension/ref" data-mw="{"name":"ref","attrs":{"name":"Floyd-2008"},"body":{"html":""}}" about="#mwt13" class="reference" id="cite_ref-Floyd-2008_6-0" rel="dc:references" data-parsoid-diff="{"id":498826,"diff":["inserted"]}"><sup href="#cite_note-Floyd-2008-6">[1]</sup></span>There are 37 such <ref> related error entries in the last 24 hours. CX used to have bugs with this and I assumed these were all CX, but I now see that this could also be from VE edits.