I don't want help on this, but figure it is worth throwing out there for future reference (in case it doesn't change).
I fired up the older of my two test Windows 10 VMs tonight and got it all updated to the current Fast ring build (10130). I'd never launched Spartan on this VM at all. After it updated and whatnot, I launched MC. It was installed (a very old build of MC20) but never licensed.
When I entered my registration code and clicked OK, then Spartan launched, but hung. The browser window just showed the blue globe splash that you get for a second when you launch Spartan.
I closed it, and it appeared to close, but then wouldn't re-launch. Subsequent attempts to hit OK in MC's registration dialog produced no result. I eventually got a timeout error from MC.
I killed Spartan in Task Manager (which was invisible, but still running), and then launched it manually. It launched fine this time. Closed it, and then clicked OK again to MC's registration dialog (which was still up). This time, it loaded the page and everything went normally.