I had a virus scan running & a few hours later I saw that a W10 Update was in progress. After that the login problem occured. It was possible the virus scan was still running when the auto update for W10 took place.
So the problem is, I can't login to any account. After I enter my password, it just gets stuck in a "welcome" loop.
I'm hoping that I could regain control in a way that keeps my files intact.