I've seen this error pop up for two reasons - maybe more. The first one
has to do with the firewall settings on a Windows computer. The second and
more common is that the ebook you're trying to download has been downloaded
under another user's ID.
In the first situation, the firewall on your computer needs to be set so
that ADE can interact and download from the ebook sites. Sometimes, the
firewall is set to deny access. I can't be more specific than that because
I don't know anything about your computer setup. If it's a MAC, then this
situation is probably not relevant.
In the second situation, once an ebook is downloaded under another ID,
you're stuck. Someone else may have a technical solution, but it will be
dependent also on your computer enviornment.
It's easy to mess up - sometimes it's hard to fix.