BUG? Cover doesn't stick

This is a multiple parts-project: I compiled to e-book, with a cover selected. but it doesn’t go into the e-book and it seems it doesn’t stick in the pane for selecting the cover.

And: what does it mean? "First image in the front matter folder?"Erstes Bild i im Titelei-Ordner?)

It probably means the first image you have in the folder you specify as front matter in the compile settings

In the Binder: Research-> Front Matter
What image is first, there? If there isn’t one there, you’ll get nothing.
If you’re putting the cover in the subfolder E-book, you need to select “Cover”, instead of “First image in…”, from that dropdown.