"Downloading Content" and never Opens a .ACSM file in ADE 4.5, but works in 3.0

I had a similar problem, where ADE 4.5.9 wasn't working for me. I tried everything I could think of. I tried reinstalling, I tried rolling back to 3.0, I deleted the Adept key and tried logging on again, etc. What finally worked for me was to purge my computer of everything Adobe and starting over.

I first uninstalled everything with Adobe in the name (including flash player and Adobe Acrobat DC). Then I went to REGEDIT, and under HKEY_CURRENT_USER, software, I exported the entire Adobe key (just in case I messed it up), and deleted it. I then went to my C:/Program Files (x86) folder and deleted the adobe folder there. I then went to my Documents folder and renamed the "My Digital Editions" folder so Adobe Digital Editions wouldn't point there.

Finally, I re-installed, re-authorized the new program, and the next book I tried to open worked. Whether that's a permanent fix, or something I'll have to do again, I don't know... but it works for now.

I don't know if this will work for you, but I found I had to open ADE 4 first and then download. Whereas for ADE 3, it would automatically open.