This error message can be difficult to diagnose as there are actually a few causes for this error. Ultimately it means that our fulfillment server is having trouble authorising your download request, but there are many reasons why this may be the case.
Most of them will be out of your control. The one thing that you can do to cause this error is to try editing the URLLink.acsm file. If you have accidentally edited the .acsm file in this way you need to delete it and download a new one.
The other more likely cause of this issue is there is an error in the (EPUB) ebook file itself that causes the server to give an error rather than supply the ebook for download. If this is the cause of the issue we need to fix the ebook file to resolve the issue.
Then there are a few other things that can cause the issue but they are less likely.
If you have tried downloading a new .acsm file and the error persists please contact our support team. Please state the error, and which of the ebooks in your account give this error. It may take a little while to find the cause of the issue but we will definitely resolve it.