In an updated advisory, Microsoft writes under the Windows 10 known issues:After installing this update, some users can not set Win32 program defaults for certain app and file type combinations using the Open with... command or Settings Apps Default apps.
Microsoft support owes these users an apology as the activations were legitimate, it was Microsoft's servers that were wrong. The message also gives users the option of purchasing a new key to reactivate their copy of Windows 10 Pro, despite users having a genuine Pro key. "In the meantime, they can continue to use Windows 10 Pro as usual". Microsoft is aware of the issue and should have a fix ready to go soon.
The agent said some Windows users might be receiving messages saying Windows is not activated and that this was due to "a temporary issue with Microsoft's activation server", which would be fixed in "one to two business days".
The vast majority of issues reported so far appear to be from users who upgraded from a previous version.
Microsoft has just released an Emerging issue announcement about current activation issue related to Pro edition recently. Microsoft has since reporetdly acknowledged the problem, explaining that the issue is with its activation server. If needed, you can try your luck with Microsoft Support to see if they can issue a replacement Pro key but it shouldn't take long before Microsoft corrects the error.
Since then, multiple reports of bugs in the Windows 10 October Update have emerged, including an issue handling.ZIP folders and a bug that caused Task Manager to report the incorrect CPU utilization.