Since this is an EPUB issue, I'm moving this to the InDesign EPUB forum.
I am experiencing the same issue. I am exporting a fixed layout ePub from InDesign CC and have tried the various options for the covers (rasterize first page and choose image) but both do not display on my iPad mini's ADE library.
Any ideas would be helpful.
P.S. StefanVenter21, have you tried using the 'rasterize first page' option? On what device are you testing on?
The cover for my fixed layout book has appeared in ADE 4.0.3 on iPad after exporting it as a book in InDesign.
Here is what I did:
1. Create a book
2. Add documents
3. Export Book to ePub
4. set your cover image in the ePub export options
I hope this helps.
A very belated, Thanks!