@lmshk, thank you very much. That is a magic trick. It solved the problem on my laptop (the one that was still rendering the static Html file OK). As far as the other computer is concerned, I will have to wait until Monday. But I guess the problem has been solved.
Nevertheless, there seems to exist a conflict between HTTP and Pluto, because I tried to re-install Pluto, PlutoUI, and HTTP and there were a lot of problems.