Windows 10 (Bootcamp) not activated in Fusion 8

[PositronicBrain]

I upgraded my Windows 8.1 Professional, Retail Edition, which was installed in my Bootcamp partition without issues, to Windows 10. The upgrade activated correctly.  However, recently if I boot this partition inside Fusion after a while Windows displays the "Activate Windows" message. At boot and for the first hour or so the OS will act as if it was registered, and the message will disappear when I reboot.  The message won't show up if I boot the Win10 partition natively.

Any suggestions on how to fix this?

I'm using Fusion 8.1.0 Pro in El Capitan 10.11.3

EDIT: OK, I am an idiot. It doesn't "become" deactivated - it is deactivated all the time when I boot in Fusion. I was just missing the activation message (this time it is downright subtle compared to previous versions). Windows 10 remains activated when I boot natively, though. Any tips?

Message was edited by: Positronic Brain

[MacsRule]

I'd suggest checking out this discussion of the issue. I'm not sure if there really is a solution in there but it's an interesting read.

[PositronicBrain · Accepted solution]

I went into that discussion as MacsRule suggested and I think it's fixed.

Short version: Use your Windows 8.1 retail key to register Windows 10.

Full version: I read the suggestion of using the Win 8.1 retail key, so I decided to try. I booted Windows in Fusion and opened the Activate dialog, then I entered the key. But then I had a brain fart and thought "Wait, I should be activating this booting natively" and I canceled the operation, and I got a message saying the activation had failed. It looks like even if you cancel the key is checked by Microsoft, though, because to my surprise Windows activated. I've booted through Bootcamp and through Fusion several times now and it is activated in both sides.

Thanks for the help!