Leonardo_Y,
thank you for your reply.
Unfortunately, the solution quoted in your answer didn't work for me. Every recovery tool I tried to use requested a password, which I cannot recall.
I got my notebook 3 months ago with preinstalled Windows 10 for Education.
The problem occured 3 days ago after I switched in safe mode (the reason was that my antivirus program didn't respond to the scan command)
I kept software, incl OS, regularly updated.
I used the Microsoft account only during first log-in in my notebook. Then I set a PIN, which was the only identification mode required.
Today, as I was trying to resolve the issue, I noticed that a few times just before the "Something happened and your PIN isn't available. Click to set up your PIN again." message appeared, a "New User" screen flashed for less than a second. Furthermore, a few times, after I clicked on the tab "Set up your PIN", a new frame appeared which stated that I have to install an app to proceed with this task and asked whether I want to search for the app in the Store. I tried both options (yes, no) with no results: the screen ("Something happend ....") reappeared.
I hope this new piece of info will get me closer to the solution with your help.
Kind regards
FV_99