I purchased a Lumia 950 XL from the Microsoft Store, and Excel worked fine. I have an Office 365 Home subscription and it worked great. It came with the stock build of W10M. I just upgraded to .107 via the Windows Insider app, and now Excel complains that I need to have an Office 365 subscription to use it. I do have a subscription using the Microsoft Account I'm signed into the phone with, and it worked with the original build. Any ideas?