Wow, I found the answer in a Q&A for another product. Went to My Products, Studio One is shown. There is a small arrow to the right of that listing. I clicked it - it is a "refresh". After the twisty finished, I went through the steps to click Download and it worked. I haven't gone home to install it yet, but I am optimistic.
UPDATE: no go. Now I get the "license isn't issued to this computer". Opened ticket with Presonus.
Jeez. I tried some troubleshooting while waiting for a ticket response - Tried removing the activation, re-adding it, and now I'm back to the Access denied in Safari and FF on Mac OS, and a 403 error in IE11 on Windows 10. The refresh trick hasn't worked thus far. If I get an answer, I'll keep you posted