Sometimes ADE gets its registration/activation confused and in a semi-authorized state. Unfortunately, it often then gives misleading error messages about what is wrong. Try completely removing any authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac). Restart ADE, and then reauthorize with your Adobe ID.
Also, in case you are thinking of reading borrowed library books on your Sony,
there is a bug in ADE2.0 that stops most Sony readers working with Overdrive library books.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
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