Sometimes, there are issues communicating with the Adobe license server when you try to return a title in Adobe Digital Editions (ADE). 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.
Please contact your library's support team to have them remove the title from your account for you.