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That error message happens to me sometimes, especially when I've dragged a bunch of .acsm files onto ADE at once (usually after I've done some batch-buying at Kobo).
Thus far, what's worked for me is to locate which one of the .acsm files which didn't go through, drop it onto ADE again, and it works the 2nd time.
Mind you, I don't know if I've been having whatever issue you've been having which may be unrelated to getting ADE to process too many things at once.
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