@ KevinH & DiapDealer: Showing the full path was a good tip.
Unfortunately, for copyright reasons, I cannot post the entire epub I scanned myself, but I hope to express myself clearly.
All book sides are all in the folder "text", chapter 1 - 129.
The CSS files under the folder "styles".
The Fonts under the folder "folder".
Here are 3 screenshots to download:
https://file.io/tlf3goCKVrWn
Sigil is confusing to me in that way, but i love this tool :-)
In older 1.X versions the clear path was analog DiapDealers message "OEBPS/Font/...", then i mean it goes with "Font/..." and now suddenly "../Font/...". But When the Path was absolut not correct, i see the message: The Path in epub not found. So something is found, but somehow with a wrong other syntax.
Why doesn't right-clicking on a marker generally go from the origin to the target in the structure as an internal link search? I don't understand why everything is made so difficult or why you have to link something freehand.
If you want me to provide more, please click more details on what exactly you want me to provide you. Thanks four your help!