Activation error and corrupted windows.old

NavyLCDR said:

Everything looks good there. I'm afraid it looks like your only options are going to be to temporarily install Windows 8 and make sure it is activated correctly, then go back to Windows 10 and you should be able to use the same product key that activated Windows 8 to activate Windows 10 - or you can do the upgrade from 8 to 10 again - or you can capture the genuineticket.xml file in Windows 8 to activate Windows 10 with. Or purchase a product key for Windows 10.

I am not sure if deleting the Windows.old.000 would then allow you to revert back using Windows.old or not.

You can do a temporary install of Windows 8 on a separate hard drive or if you have room make a partition for it on your current hard drive. Personally, if it was me, I would back up what I want to save on a USB thumb drive or USB hard drive, wipe the installed hard drive and start over from scratch with a new install of Windows 8.

Is there absolutely no other way? My laptop is still under warranty, can I ask HP to do something about it?

I just contacted MS support and they gave me a new key... Everything is solved now

Awesome! Glad it worked out for you!