I came here with a similar issue - though my computer was authorised, I kept getting this error message when trying to open a .acsm file downloaded from the library. I found this thread with some different suggestions - https://forums.adobe.com/message/5977702#5977702
Not sure if your problem is the same, but I actually managed to fix it based on the last comment on that thread. I think the issue was because I had tried to open the file before my computer was authorised. So I authorised my computer, deleted the file and downloaded it again, and made sure Digital Editions was open and authorised before trying to open the file again.
Hope this helps!
I found a solution changin the time zone of my clock