I wouldn’t say this is fixed… as it took me many hours to get the download to start but workaround.
Keep re-trying with starting Download Assistant. It updates itself and needed 3 re-tries from me today before it seemed to update itself correctly, presumably something has been changed since yesterday. Major visual difference was at the 3rd atempt it updated C++ redistributable and some other components.
Do a log in/out of your Steinberg account for good measure, I don’t know if it was relevant but I did that. Remember to click ‘always allow access’ in the Browser security pop-up, again I don’t know if this was actually relevant.
In Download assistant look at the Target folder for the downloads, it’s displayed Greyed out at the top. In my case the folder “Steinberg” in my Downloads had never been created. Folder created manually. Cancel all downloads, re-start Download Assistant. It still fails! Cancel all Downloads again but DON’T try to Install. Re-start Download Assistant again. Then I think it will work… or maybe you have to re-start Download Assistant a 3rd time… 
Can’t remember exactly how it happened now. Eventually it starts to download.