How do I find the illegal hex character that halted a MediaWiki XML conversion through Filter Streams?

If I leave the “Filter Streams Converter” page and go back to it, I end up with the session I ran before still displayed there. How do I start a fresh instance of it? What’s on the page from the old session isn’t even the full form to specify what to import. Ah, I see, it expands if I start to fill it in fresh…