Hi Jarle
I risked trying the upgrade on my main music PC and am surprised to say that it worked…
It would seem that the problem on my laptop was with the Activation Manager…
This may not help you but my suggestion would be to uninstall Cubase and start again using the Steinberg Download Manager.
I wish you lots of patience and luck, joe
Thank you very much Joe!
I will try it or maybe to use another PC to activate the license.
Thank you so much for your reply Joe2001! 
And I am very happy that you solved your problem!
You uninstalled the old Cubase, but will it detect all projects and instruments when I re install it?
Yes, that was my experience, nothing was lost,
good luck, joe
Thanks to @joe2001 I solved the same problem.
But in my case I don’t uninstalled my old Cubase, I just did an offline license activation with another PC and it’s worked
Now I have the 2 versions of Cubase (10.5 and 13) and the 13 detect and open all my projects and instruments (after rescan my folder)
Well done Florant!
My solution was an uninstall/ reinstall of Activation Manager- full details in my long post about this error,
now enjoy making music again, joe