I help customers downloading ebooks with ADE. Brave person ... Good luck
ADE messages are sometimes not quite appropriate.
I suggest trying the following in this order.
Order chosen for easiest first, rather than most likely to work first.
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(1) Remove any old authorizations (ctrl-shift-D to ADE, cmd-shift-D if on Mac).
Restart ADE, and try to authorize again.
(2) (Temporarily) disable any firewall and try again.
(3) (Temporarily) disable any antivirus and try again.
(4) Try the older but more reliable v1.7.2.
I feel like a broken record suggesting this one; but it has proved the solution in lots of different situations,
including this one. http://forums.adobe.com/message/5051168
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172