After I've entered all data in "New Cloud Connection" and cklicked "OK", there is no cloud connection at all in the "Connections" panel!
You have to SHOW us what you did and HOW you did it. You can't just say 'I tried and it did not work'.
I read lot of stuff on the Internet (e. g. Kris' blog: SQL Developer and Cloud.oracle.com) about SQL Developer and Cloud,
but all this couldn't help me: I cannot create a cloud connection.
Re-read that article carefully. Note that they did NOT just say 'if you try to connect you will succeed'. They SHOWED you, step by step, EXACTLY what to enter in every field of every screen.
That is what you need to show us: everything you entered in every field of every screen. We can NOT see what you type or the results that you get.
Yo are absolutely right, I made a video now: