Setting jetty to DEBUG shows the requests. It’s a bit hard to see, because jetty is very chatty at DEBUG.
After I found that, I originally thought that amazon is blocking jetty as user-agent and tried to set the user-agent header to a more common Mozilla. To my surprise this resulted in two user-agent headers which is obviously wrong. I then had a closer look at the other headers and found the the binding adds headers that are later added by jetty. After I removed that, the connection succeeded.
I‘m not entirely sure if this should be considered a bug in jetty. If I pass a request to the client, IMO it should not add already existing headers.