The Hyperlinks connect the Documents in the Book File, in the Preview Panel in InDesign it works perfectly, the actual Buttons don’t. Funny enough, when I export the File as a Fixed Layout ePub only the Buttons work then and the Hyperlinks don’t.
Thank you for your input, I've actually figured out a solution. Maybe this will help some other people so: I changed the Hyperlink Source in the dropdown from File to Page for all the Links. Suprisingly that fixed my issue so happy end!