@Semwize,
In addition to 3 minor problems (according to calibre's Check Book) with your epub there is one major problem which is causing so many errors in epubcheck. This major problem is that throughout the epub (but mostly in ch2.xhtml the footnote file) the HTML has >200 occurrences of <span> tags which contain <div> and <p> tags.
The good news is the simple solution (for this particular epub) is to use an epub editor to change all those <span ...> and </span> tags to <div ...> and </div> tags. You then only have the 3 minor warning errors to clear up. If you are familiar with epub clean-ups you should be able to do all this in a few minutes.
The bad news is that even once the epub is clean I don't
think the HTML footnote links are coded in a way that will cause the kepub to display them as pop-ups, because each footnote does not contain a matching backlink to where you came from.