Ok, I went ahead and upgraded to Windows 10. everything went smoothly, all my settings and installed apps preserved and work without a hitch. I am loving the interface and getting along with Cortana pretty good.

But after a while I encountered a weird issue: Appstore installed apps – e.g. Calendar, Mail etc. even Windows AppStore itself wouldn’t launch. I’d either get a cryptic error message, something along the lines “Application did not start, please contact system administrator” or very briefly a window would appear and immediately closed.

Looking into Event log was a bit more explanatory, but not too much: “Microsoft.WindowsStore_8wekyb3d8bbwe!App failed with error: Access is denied. See the Microsoft-Windows-TWinUI/Operational log for additional information.”

If you google it – you will find many possible explanations of the problem and many possible ways to solve it offered, but none of those worked for me. Finally I figured it out (and Event Log entry gave me a clue): C:\Program Files\WindowsApps folder was missing necessary permissions:

Namely, “ALL APPLICATION PACKAGES” was missing read & execute permissions on that folder (had to to uncheck “Hide protected operating system files” in Control Panel, File Explorer options to be able to see it). Once I added correct permissions – appstore apps started launching with no problems.