Maybe I got the same problem...
I bought an e-book (an .acsm-file) without having ADE installed on my desktop computer. Instead I downloaded Overdrive (i.e. the Windows 10 app). I then could open the e-book en read it.
After that I read on the internet I should have used ADE. So I downloaded ADE 4.0
When I try to read the e-book in ADE I get an error: E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER
What can I do to open the e-book in ADE???