When this problem hit me finally, deleting the files and directories mentioned in this thread got it working again for me. Prior to doing that I had already tried deleting and reinstalling both the Reader Library software and Adobe Digital Editions, with no success. I skipped to just deleting what was listed and it worked fine without having to reinstall it again. You probably won't have all the files/directories mentioned, just delete what you do have.
https://www.mobileread.com/forums/sho...&postcount=111
Edit: Just in case the above doesn't work, this might help you (it didn't for me):
https://www.mobileread.com/forums/sho...2&postcount=44