I got the same error. Check your ebook reader. I bet you are at capacity. Remove/archive some books to make room and you should be good to go.
I've got the same problems. It's decidetly not the capacity, my ebook reader ist empty and the error occurs when downloading onto the computer...
Please try re-downloading the book.
Found the root cause:
- Digital Editions attempts to download ebook to temporary file
- Temporary file (in %TEMP%\Digital Editions) cannot be created because of permissions 
- Digital Editions errors out with a not very helpful "E_IO_CANNOT_OPEN" result
- Reset the NTFS permission of the Temp folder and below to include your user, for example with this approach (using %TEMP% as folder name, e.g.,
takeown /f "%TEMP%" /r
icacls "%TEMP%" /reset /T
 The permissions for the entire Temp folder have been changed by some program, removing the user from the permission list. This may be an Adobe problem itself, as explained in this post, but has not yet been verified. Resetting the permissions makes Digital Editions work as expected, until the permissions are changed back again.
OMG!!!!!!!!!!!!!AFTER HALF A YEAR OF NOT BEING ABLE TO DOWNLOAD BOOKS!!!!!!!!!!!!!! THANKS A LOT!!!!!!!!!
I've just installed ADE 4.5 but get this error message when I try to open it (ADE) "Unexpected error
Unexpected error occurs:
Is this a permission error too? I've just had Windows 10 installed (by Microsoft) and have had a host of permission problems. I think the issue is that Microsoft did an overlay of 10 on Windows 7 instead of completely uninstalling Windows 7.
I'm really glad there is a solution to this problem since it seems to much like the common cold (no cure!)
However, I'm not that tech savy and when you say:
- Reset the NTFS permission of the Temp folder and below to include your user, ....
I have no idea how to do this.
Would someone please give me some for Dummies level instructions on how to do this?
It would be so much appreciated!!
You are awesome! This worked. Just typed run from my 'windows square' typed in your commands (including the quotation marks) and next thing I new my Adobe Digital Editions Software was working again. (I hate Adobe Digital Editions...won't ever buy another book on that software as it SUCKS, but at least I can read to one book I made the mistake of buying on that software). Thanks so very much!!!!!
Just get a refund from google books.
Thanks a lot for the idea to check the properties of the temp folder : I just had to cancel its "read only" property. (The two suggested commands had no visible effect and led to the same error message).