I too am having the same problem. I have a new hard drive that was imaged from the old one and I'm trying to side load a book to my Nook Simple Touch.. I first installed ADE 4 then borrowed and downloaded a book from Overdrive. I tried a few times to delete and reauthorize both the computer and the Nook and each time I get an authorization error when I try to side load the book. I then found on the Nook forum that the Simple Touch and ADE 4 don't get along. So I uninstalled ADE 4 and installed ADE 3.
I then followed this recommended procedure found on the forum: I deleted the Digital Editions folder from my Nook, launched ADE 3 and authorized the computer, then connected the Nook. For a few seconds, hovering the mouse over the setting icon for the Nook displayed everything on the context menu grayed out. But then it showed the context menu and I was able to display the Nook's authorization, which was the same as the computer's. Still get the same error side loading the book. Deleting and reauthorizing doesn't help. And the Digital Editions folder is still missing from the Nook.