Sometimes ADE gets registration confused.
This can often (not always) be fixed by completely de-registering using ctrl-shift-D to ADE (cmd-shift-D on Mac).
Then restart ADE.
Then tray again to authorize.
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Also, there are lots of bugs in ADE 2.0.
Problems can sometimes be resolved using the older but more reliable ADE1.7.2.
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