Originally Posted by Jim Lester
Most likely it's a file (or files) on your Kobo that ADE is trying to read. I would start with seeing if there is a Digital Editions directory on your Kobo and renaming it (in particular a really bad manifest.xml may be causing problems). After that it may be a file that is labelled as an epub or PDF that actually isn't.
Thanks Jim - that makes much more sense than the reply Adobe gave me! I am about to get a new ebook reader but I would like to use the old one at home for other family members, so perhaps a good clean-up of old files etc might get rid of this problem.