SSH connection to jenkins server
Your configuration refers to a file that is local on your system, but does not exist on the jenkins server.  The error message is trying to tell you this.  /c/Users/D35N/Downloads/GitHubProjeckt/Automation/GitHubProjeckt/newharbortestkey is not on the jenkins server.  You need to make this file available on the jenkins server by copying it up there, and it probably shouldn't be in your personal Downloads folder.
You may want to configure your script to look for a series of files and use the first one that exists.  This will let you have the file at different locations on your development box versus the server and it will still work in either.