You are trying to resume a download which has been protected with a username and password. In this case, you need to start the download from the beginning while browsing the homepage. In this way you will keep the cookie active on your computer allowing you to download items. I believe this has to do with cookie and their expiry date.
Keep the homepage of the download active perform the download in the same time until it finishes.
Of course, there is the possibility that the server from where you download does not support download managers which in this case you should download the file with your browser.