Update: I called my ISP some six hours ago. Their tech said he can't do anything because it's not their network. However, he promised to forward the message. Which was just what I expected, knowing that an ISP will be listened much more carefully than a single end user.
I don't know what has been done, the Ping Plotter output still shows a 100% packet loss for Hop #9 but the forum now seems to work much better than it used to.