Can you please confirm the ADE and OS version you are using?
Can you please try to re-authorize ADE and try again? If you still get the issue please try to download .acsm file again and check.
Windows 10, ADE 4.5.6
I've tried to re-authorize and re-download and nothing has changed.
Google book-support has aswered me that Adobe does not support the embedded font encryption format (in italian: formato di crittografia dei caratteri incorporati) used for that book.
There is any chance to finde a way to solve this incompatibility? The ISBN code is 9788830441019
Operating system is OSX 10.11.6
ADE: 4.5.4 (but have also tried ADE 3.0)
Have tried reinstalling ADE; reauthorizing Adobe account; redownloading
.acsm file (both as an epub and as an pdf). Nothing here has worked.