In v1.7.2, look under menu/Library/About Adobe Digital Editions (or menu/Reading/About Adobe Digital Editions if in Reading mode).
In v2.0, under menu/Help/About Adobe Digital Editions
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There are lots of bugs in ADE2.0, though I haven't heard of that particular one. 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