Browser problem "book-manifest"/can't read ePub // standalone content server on Linux

Dear all

I´m a total novice when it comes to Linux - so sorry for all my stupid questions.

=> background information
- Stand-alone calibre-server running on Linux/Ubuntu Server (MacMini).
- Access to ePub via Internet / WLAN possible
- possible to upload and change ePub information


==> Problem
Can't read ePub in browser-application. I always get the same message: I do have NO idea of how to solve this problem. As I am a novice to Linux I would like to ask to answer as precisely as possible --- I would like to directly copy/past your instruction....

Thanks for your help.
I very much support the idea of sharing epubs by using calibre - calibre server is a perfect tool to do so.

Henning Strodthoff