Sometimes, there are issues communicating with the Adobe license server when you try to return a title in Adobe Digital Editions (ADE). When this happens, you'll get an error like "E_BAD_LOAN_ID" or "E_LIC_ALREADY_RETURNED" in ADE. This typically means that the book was successfully returned, but it didn't get deleted from your ADE library. It can also happen if you attempt to return a title after it's been returned.
To resolve the error, visit your library account to see if the book is still checked out to you. If it is, you can return it from there, then remove it from ADE (without returning it).