So it works now. Difference was, do not use com.mendix.storage.s3.Region. Instead use com.mendix.storage.s3.EndPoint=s3.eu-central-1.amazonaws.com. Now it works perfectly.
Hi Chrisitan, that is correct when hosted on the Mendix Cloud it will store all file documents on to the S3 bucket which is managed by Mendix on your behalf.
What is the purpose of wanting to use their own S3 bucket?