Hi all, 
Update:
seems I could solve the mystery now.
It turned out I was still logged in at itsfoss.com as I never could log out from it any more.
As a result logging in to the forum site was easy as I just had to login from there.
Well, there seems to have been a change in the logout procedure from the main page:
On itsfoss.com there´s no longer a logout button.
But: on the membership page (on It's FOSS) there is such a “button”: It´s called “Sign out” and it´s hidden beneath the personal user´s icon.
Now I can log out from the main page again.
So the signing in/out procedure is doable the way it used to be again, with a minor modification as far as logout from the main page is concerned.
Instead of
itsfoss.com
it´s
https://itsfoss.com/membership/
now.
Funny thing though that we hadn´t been informed about that change. 
Never mind. Everything´s clear now, I guess.
Many greetings from Rosika 