This Secured Download Manger relies on the internet (ie. IE9 prior to Modern Windows), it doesn't "unpack" and make the ISO available unless the ".SDC" can be downloaded though one of Microsoft's browsers (is your default browser). If the download is broken then it just sits there, as the name suggests.
I have this problem and the answer is to use Windows Core 8 or System Builder's disk (where it is for a whitebox PC, instead, and just enter the OEM's product key where it is not). Then use the core key or OEM's key, because that's why you won't need the time-sensitive license.
Nothing else can be done, that's how I licensed windows and all my Windows 8 PC's worked fine re-imaged and all. Dreamspark does give changes, usually about two per key, and then which doesn't work grays out.