Thanks for your response, Martin!
I tried pressing
Sorry for my blindness! And thanks again for your help.
I tried pressing
Ctrl+Shift
, but nothing changed; PuTTY didn't open. Nevertheless, when I regularly open putty from WinSCP and I read all the messages (first time I really see it), I see, that the key format is too new. I also read how to fix that by converting the private key in PuTTYgen.
Sorry for my blindness! And thanks again for your help.