Resolved it by activating the Bitstamp API Key. Yes, my bad. Obviously, Bitstamp does not activate new API Keys by default. A documentation for the setup of all supported exchanges and better error handling would be great.
I can take care of the documenation part if there is enough interest.
PS.: I am currently trying to get 796 working, but struggling. Does anybody have a working 796 integration?
Just checked this thread, but glad you got your API key working. I haven't tried 796 integration myself but it is most likely possible. Perhaps dev will add it in the future.