This error message is normally received when attempting to authorise Adobe Digital Editions or when attempting to download in ADE 1.7 or 1.7.2
When you receive the error there should be a button to 'Try Again'. So the first thing to try is to click the button and hope it works on the second attempt.
If it does not work after a couple of tries then the next thing to do is to deauthorise ADE, then rebuild (delete) the activation file, and then re-authorise ADE.
First deauthorise ADE by following these instructions.
Even if you manage to deauthorise within the ADE software you should then also follow the Windows/ Mac OS specific instructions to delete the registry key (Windows) or activation.dat (Mac).
Then once that has been completed you can reauthorise using these instructions.
Finally you need to delete the ebook from your ADE bookshelf and redownload it from your eBooks.com account now that you have your ADE authorisation in order.