This error message can be caused by two different issues.
The first one is if you rename your URL_Link.acsm file before attempting to open it in Adobe Digital Editions (ADE). If you have renamed the file please delete it and then download a new .acsm file and try opening it immediately without renaming it.
The second issue can occur if there was a problem importing the ebook file in to our file server. This then means when ADE opens the acsm file and tries to request the ebook from our server it cannot.
In order to fix this we need to reimport the ebook file and reset your download licence.
Please contact our support team stating the error message E_ADEPT_DOCUMENT_TYPE_UNKNOWN, also state which book you are having this issue with. Also if the book is available in more than one format (PDF and EPUB) then please state which format you were attempting to download (if you don't know, that is ok, we can check both).
Once we have fixed the issue we will ask you to clear your browser's cache / temporary internet files and retry the download.