Rest API Bad Request

Hi,

thanks for providing the rest api, very useful!

However, when following the guide in the first of your rest api posts https://community.aras.com/b/english/posts/tech-tip-using-the-aras-restful-api

I get a 400 Bad Request with the response 

"message""Value cannot be null.\r\nParameter name: loginName"
I did put database, authuser, authpassword in the header as described.
I use postman and the server is V12SP2
Any ideas on how to solve that?