You're going to have to answer the question yourself, unfortunately. I can
explain what the message means, but I can't do the detective work to see
what happened in the past.
This message is generated when the information attached to the
document/ebook shows that "User A" downloaded it to his/her copy of Digital
Editions is different from what's in place at this time. There are a number
of things that could have happened, but right now, "User B" is trying to
download that document/ebook.
There's no easy fix - especially if the document/ebook is DRM-protected, as
many of the ones we buy are. Once you find out who did what, then you could
contact the source and see if they'll work with you to get another copy
downloaded to your copy of Digital Editions.
What user account is it referring to? It doesn't matter whether it is an epub form Amazon or Barnes and Nobel, or anyone else. I get the same message. Is it my ADE account? I uninstalled it and reinstalled it and re-authorized it to no avail.