The same problem here, OS kinda bugs after several days of usage, I am starting to have some problems.

- The start opening is delayed, its kinda annoying, and yes, that 0.5-1 sec is pretty long, ...

Yes I can confirm that. If you do clean install, windows 10 will not accept your previous legal win 7, 8 or 8.1 product key.

Can you please confirm, I need to "Upgrade" from previous activated copy of windows (win7,8) in order to have win10 activated.
So I cannot clean install, format drives etc. and boot USB win10 which...

It seems to me the word "upgrade" is important. If you do clean install (boot from usb, format drives etc.), the windows wont remember your previous key and wont activate.

Well, I have photo of product key label taken, so its proof error was not on my side and I am eliglible for win10 upgrade.

Well, I used MediaCreationTool to put win10 into USB, booted into bios...

Same problem here, clean installed from legal Windows 7 HP, but activation key doesn`t work. Error code is 0xC004F050

edit: could it have to do something from MediaCreationTool where it asked...

So if I understand it right, If I do clean install from USB (I downloaded win10 from website since I didnt want to wait for upgrade notification), my key will not work? I have legal windows 7 HP...