Possible answer is that ADE has its authorization confused: it often gets confused, and then gives inappropriate error messages. Try
- ctrl-shift-D (cmd-shift-D on Mac) to ADE to remove any old authorization
- restart ADE
- try again to authorize with your Adobe ID.
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If that doesn't work, try using the older but more reliable ADE 1.7.2 instead.
(It would be difficult to be less reliable than ADE2.0)
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