After some YouTube comments update, when you click to show all comments in a comments tree or hit reply under a comment, YouTube reloads the page for some reason.
It turns out this only happens when you open a non secure, i.e. http connection to YouTube. You can watch the video, but hitting the reply button, it reloads the page using secure HTTPS protocol. That means that if you browse YouTube in HTTPS from the start, the page won’t reload because you are already in HTTPS. Too bad when you click a comment from the comment notifications, it automatically opens the video in plain HTTP. You may sit and wait while Google fixes this error, or you may install a browser plugin and configure it to automatically rewrite all HTTP requests to HTTPS when browsing YouTube. A fine example of such plugin that forces HTTPS where possible is HTTPS everywhere, available for all major browsers.