It is working now. To finish this case here:

The API docs are false. In the docs is written that you must login with "curl -sv --cookie "memberSession=SESSIONIDHERE" -H "Authorization: Bearer..." this returns some data and here is the error.

In the docs is written that you can create an entry with "curl -s --cookie 'memberSession=SESSIONID' -H 'Authorization: Bearer..." but you have to use "curl -s --cookie 'memberID=1' --cookie 'memberSession=abcdefghijklmnop' --cookie 'memberPassword=12345abcd -H 'Authorization: Bearer..."

So in the cookie memberID and memberPassword must be set also out of the response from the login.