ADE 4.5. unable to open epub file

I just buyed and downloaded a new DRM unprotected epub book. ADE 4.5.10 (Windows 10) accepted to load the book in the library but refuses to open it (Message says in french "Des erreurs ont été détectées dans cet élément" which translates to "errors detected in this item"). All previously bought books, protected or not, still work fine. The book opens correctly in Calibre or in my digital book reader after transfer over USB. Looking in the forum, I tried every possible way to correct the problem, including changing the system date to a prievious day, uninstalling/reinstalling ADE, modifying the file date attributes, deleting and reimporting the book, rebooting etc. Nothing worked. Installing the backlevel ADE 3.0 however works fine and the book opens correctly. Reinstalling ADE 4.5. won't change however. Displaying the permissions info from ADE shows: Licensed to : Unknown, and all 3 operations allowed to any device, like all other unprotected epub books. Of course my computer shows in ADE authorized to my email and corresponding adobe-ID. 

This leads to stupid loss of valuable time. Please correct this strange behaviour asap.