Meanwhile I figure out the Problem, it was related to Windows App - AccuWeather. I uninstalled it and error is gone.
I have big issues with Windows Apps, seems there a general mismatch Permissions related with Windows Apps.
It also affects very much the Boot performance.
I have been winnowing down errors in my event log and was succeeding until today.
Installed update KB3194496 (I am not on any preview streams).
And got the following error:
The Appx operation 'RegisterPackageAsync' on...
I'm doubtful that this is really a Win10 message but what idiot, other than MS, would think a message saying only "Software download complete" is OK? I'm serious-it pops up from my system tray & that's all it says other than the choices 'Install...