@kaniini  Thank you, after several attempts I figured out the correct full URL for the "whoami" API endpoint:
https://pleroma.site/users/AndStatus/inbox - and I got JSON response. Will parse it and return to You to move further, if you don't mind :-)

However, this URL, looking like an actor's inbox, is conceptually incorrect for the "whoami" call as even # spec says: "The inbox is discovered through the inbox property of an actor's profile".
This is why the "whoami" call should be the same for every user, and not the "inbox" of a concrete user/actor.
I think :-)

This is "whoami" call where the authenticated Actor receives his/her profile information, including URLs of endpoints!

?!