I too have had several issues with version 4.0 and 4.5. I resolved them only by uninstalling the newer versions and going back to 3.0, which, in my opinion, is the only really stable one.
I find also unacceptable the refusal to offer support for a product that although free, is essential to use a purchased item as an ebook. If they don't want to give support to ADE they shouldn't make it the only DRM service for ebooks available...
I had a similar set of problems, chat won't help because it is not a paid product issue I guess. I posted and received nothing from the forum and tried multiple different solutions on OS X 10.11 and 10.12. No dice.
Downgrade to v3.0 and suddenly it all works. Sloppy, sloppy coding and I wish there were options for other DRM software that I could choose from.
(For any future download issues, I had to dig on the Net to find the link for v3.0 here - Download | Adobe Digital Editions Home ).
Through much uninstalling, reinstalling, etc. I am down to only one problem - I get 'User not activated' when I try to open a book I've downloaded to my nook. I even went to the library and got a new library card so I would have a different account. I am currently using ADE 3.0.