Thanks! Found it. Still mystified. I can't find any unclosed element:
<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="BookID" version="2.0">
<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns
pf="http://www.idpf.org/2007/opf">
<dc:title></dc:title>
<dc:creator opf:role="aut"></dc:creator>
<dc:language>en</dc:language>
<dc:identifier id="BookID" opf:scheme="UUID">a0d1fd34-da99-4a38-bdca-773188174932</dc:identifier>
<meta name="cover" content="olgacover800.jpg"/>
<meta name="Sigil version" content="0.2.4"/>
</metadata>
(I spaced them out in order not to get them all run together, and I removed the book title and author. I make dc:identifier as line 7.)