How to debug "Exception Error Messages" of fulltextsearch:index?

For eaxmple

occ fulltextsearch:index

breaks with a message like this

┌─ Errors ────
│ Error:     14/14
│ Index: files:445719
│ Exception: Elasticsearch\Common\Exceptions\BadRequest400Exception
│ Message: Markup im Dokument nach dem Root-Element muss ordnungsgemäß formatiert sein.

I have Kibana installed and the console open at http://localhost:5601/app/dev_tools#/console

What do I have to enter here to find out the incorrect document?