[KevinH]

Please provide the full epubcheck error message.

As for the nav it is included in the spine itself and therefore should appear in the epub as the last part.

The pagelist has the "hidden" property, so even if the nav is shown as part of the book the pagelist will not be shown.

Your nav and opf themselves look reasonable.

Different reading systems handle pagelists in different ways.

Other things:

The first dc:language is German, so that should be the default language of every xhtml page unless a lang or xml:lang attribute exists on the html tag in those files. Your nav is written in English but no language attributes are provided.

No empty lines or added space should exist before the xml header in your nav file. This may be the cause of the epubcheck error.

The toc landmark in the nav should point to the nav itself or to a separate non-nav table of contents that is part of the spine, this is typically not the back cover.

If you are not making an epub3 be backwards compatible with older epub2 only readers you should remove the entire toc.ncx as some readers will default to it if exists. Similar issues exist with having a guide in the opf if it is meant for epub3 only. Only add a toc.ncx and opf guide if you want to make the epub3 be backwards compatible with older epub2 only readers.

Having the entire text body inside a single xhtml file is simply not an acceptable design unless there really are no chapters or section breaks.