This error occurs when a user tries to fulfill an eBook using a download link that was previously used with a different Adobe ID.
For example, let's say your user purchases the eBook titled 'The Art of War' from you and receive their download link for it. They use the link to download and open the eBook in Adobe Digital Editions with Adobe Id 'firstname.lastname@example.org'. A few weeks go by, and the customer also wants to download the same eBook onto their laptop. They use the same download link they used before, but this time with Adobe Id 'email@example.com'. That's when they'd receive this error. To correct this problem, they'd have to use 'firstname.lastname@example.org' as before.