[Leonatus]

I have produced a lot of epubs by now, in Sigil as well as in Calibre. But this time, I'm coming to my limits. When inserting images or fonts, I receive - in Sigil and in Calibre - those messages:

WARNUNG: Die Datei 3Sternchen.png ist nicht referenziert****[3Sternchen.png]
WARNUNG: Die Datei Exlibris_Bogen_2020.jpg ist nicht referenziert****[Exlibris_Bogen_2020.jpg]
WARNUNG: Die Datei FoglihtenNo04-070.otf ist nicht referenziert****[FoglihtenNo04-070.otf]
WARNUNG: Die Datei W3Ccss.png ist nicht referenziert****[W3Ccss.png]

But in the OPF, they are noticed:

<?xml version='1.0' encoding='utf-8'?>
<package xmlns="http://www.idpf.org/2007/opf" version="2.0" unique-identifier="uuid_id">
  <metadata xmlns:calibre="http://calibre.kovidgoyal.net/2009/metadata" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:opf="http://www.idpf.org/2007/opf" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <dc:title>Die Narrenburg</dc:title>
    <dc:creator opf:role="aut" opf:file-as="Stifter, Adalbert">Adalbert Stifter</dc:creator>
    <dc:date>2013-10-27T23:00:00+00:00</dc:date>
    <dc:contributor opf:role="bkp">calibre (1.8.0) [http://calibre-ebook.com]</dc:contributor>
    <dc:identifier id="uuid_id" opf:scheme="uuid">da7f9ece-e270-4413-a3b7-3376277e4231</dc:identifier>
    <dc:date opf:event="modification">2020-09-09</dc:date>
    <dc:description>&lt;div&gt;
&lt;p&gt;Rechtschreibung aktualisiert.&lt;/p&gt;&lt;/div&gt;</dc:description>
    <dc:language>de</dc:language>
    <dc:identifier opf:scheme="calibre">ba81fe0c-d397-4aa5-bf1a-7622295703f2</dc:identifier>
    <meta content="Narrenburg, Die" name="calibre:title_sort"/>
    <meta content="2013-10-28T17:33:58.150000+00:00" name="calibre:timestamp"/>
    <meta content="{&quot;Adalbert Stifter&quot;: &quot;&quot;}" name="calibre:author_link_map"/>
    <meta content="1.3.0" name="Sigil version"/>
    <meta name="cover" content="Cover.jpg"/>
    <meta name="calibre:user_metadata:#hyph" content="{&quot;is_csp&quot;: false, &quot;is_multiple2&quot;: {}, &quot;is_editable&quot;: true, &quot;is_multiple&quot;: null, &quot;search_terms&quot;: [&quot;#hyph&quot;], &quot;#value#&quot;: null, &quot;column&quot;: &quot;value&quot;, &quot;rec_index&quot;: 22, &quot;datatype&quot;: &quot;enumeration&quot;, &quot;#extra#&quot;: null, &quot;colnum&quot;: 1, &quot;name&quot;: &quot;Silbentrennung&quot;, &quot;kind&quot;: &quot;field&quot;, &quot;category_sort&quot;: &quot;value&quot;, &quot;is_custom&quot;: true, &quot;is_category&quot;: true, &quot;table&quot;: &quot;custom_column_1&quot;, &quot;link_column&quot;: &quot;value&quot;, &quot;label&quot;: &quot;hyph&quot;, &quot;display&quot;: {&quot;description&quot;: &quot;&quot;, &quot;use_decorations&quot;: 2, &quot;enum_values&quot;: [&quot;ja&quot;, &quot;nein&quot;], &quot;enum_colors&quot;: []}}"/>
    <meta name="calibre:user_metadata:#pages" content="{&quot;is_csp&quot;: false, &quot;is_multiple2&quot;: {}, &quot;is_editable&quot;: true, &quot;is_multiple&quot;: null, &quot;search_terms&quot;: [&quot;#pages&quot;], &quot;#value#&quot;: 126, &quot;column&quot;: &quot;value&quot;, &quot;rec_index&quot;: 23, &quot;datatype&quot;: &quot;int&quot;, &quot;#extra#&quot;: null, &quot;colnum&quot;: 3, &quot;name&quot;: &quot;Seiten&quot;, &quot;kind&quot;: &quot;field&quot;, &quot;category_sort&quot;: &quot;value&quot;, &quot;is_custom&quot;: true, &quot;is_category&quot;: false, &quot;table&quot;: &quot;custom_column_3&quot;, &quot;link_column&quot;: &quot;value&quot;, &quot;label&quot;: &quot;pages&quot;, &quot;display&quot;: {&quot;number_format&quot;: &quot;{0:,d}&quot;, &quot;description&quot;: &quot;&quot;}}"/>
  </metadata>
  <manifest>
    <item id="cover.xhtml" href="OEBPS/Text/cover.xhtml" media-type="application/xhtml+xml"/>
    <item id="Titel.xhtml" href="OEBPS/Text/Titel.xhtml" media-type="application/xhtml+xml"/>
    <item id="Impressum.xhtml" href="OEBPS/Text/Impressum.xhtml" media-type="application/xhtml+xml"/>
    <item id="Kapitel_1.xhtml" href="OEBPS/Text/Kapitel_1.xhtml" media-type="application/xhtml+xml"/>
    <item id="Kapitel_2.xhtml" href="OEBPS/Text/Kapitel_2.xhtml" media-type="application/xhtml+xml"/>
    <item id="Kapitel_3.xhtml" href="OEBPS/Text/Kapitel_3.xhtml" media-type="application/xhtml+xml"/>
    <item id="Section0001.xhtml" href="OEBPS/Text/Section0001.xhtml" media-type="application/xhtml+xml"/>
    <item id="Section0002.xhtml" href="OEBPS/Text/Section0002.xhtml" media-type="application/xhtml+xml"/>
    <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
    <item id="footnotecss" href="footnote.css" media-type="text/css"/>
    <item id="id1" href="kte-css/hyphenation.css" media-type="text/css"/>
    <item id="page_css" href="page_styles.css" media-type="text/css"/>
    <item id="css" href="stylesheet.css" media-type="text/css"/>
    <item id="Cover.jpg" href="Cover.jpg" media-type="image/jpeg"/>
    <item href="3Sternchen.png" id="id" media-type="image/png"/>
    <item href="W3Ccss.png" id="id2" media-type="image/png"/>
    <item href="Exlibris_Bogen_2020.jpg" id="id3" media-type="image/jpeg"/>
    <item href="FoglihtenNo04-070.otf" id="id4" media-type="application/vnd.ms-opentype"/>
  </manifest>
  <spine toc="ncx">
    <itemref idref="cover.xhtml"/>
    <itemref idref="Titel.xhtml"/>
    <itemref idref="Impressum.xhtml"/>
    <itemref idref="Kapitel_1.xhtml"/>
    <itemref idref="Kapitel_2.xhtml"/>
    <itemref idref="Kapitel_3.xhtml"/>
    <itemref idref="Section0001.xhtml"/>
    <itemref idref="Section0002.xhtml"/>
  </spine>
  <guide>
    <reference type="cover" title="Cover" href="OEBPS/Text/cover.xhtml"/>
  </guide>
</package>

I have no idea how to continue. Waht is wrong, please?
(My system is Windows 10, 64 bit, Sigil V. 1.3.0, Calibre V. 4.23)