>> when you log off ad re-log on the issue disappeared
That's true. But just switching to another user and logging back to the original user, does not make the issue disappear. It's difficult to say what to look for in the LOG, but I did not find anything special, and this makes me believe it's due to some weird
interaction between programming elements.
>> create another local admin account and then logon to the computer to check the result
I already have 2 admin accounts (MS accounts), and it may happen with both. Currently, the Windows Update window has changed (*).
>> Run DISM /Online /Cleanup-Image /RestoreHealth , CHKDSK, etc...
I ran all these before , for other reasons. And after I ran them, the issue happened again 5/june/2015, in the first hour.
(*) Currently, I got the Download Windows 10 logo on my task bar, and this has changed the Windows Update window, which may result in the issue in question being automatically swept off. Only time will tell.