Sometimes ADE gets confused and in a half-authorised state.
cmd-shift-D (ctrl-shift-D on a PC) should deauthorize competely. Restart ADE, then try to authorize again.
There are bugs in ADE2.0 (on PC at least) that prevent it working for Overdrive library books with PRS-350; same bug may apply to PRS-650.
That can be resolved by ininstalling ADE2.0 (which also has lots of other bugs) and installing the much 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