Notifications about replies from the bell icon in the green menu and email notifications of replies are currently broken, so you'll see '404 page not found' errors if you click on a link in an email or from the Notifications menu. I've reported the bug directly to HU (the Help email system is also broken!) so I expect this problem will be given urgent attention Wednesday morning UK time.
Meanwhile, please use Newsfeed to get to any replies. Newsfeed entries can be one of two types. When a new post/question is submitted, there's just one entry containing a link to the new post. Whenever someone replies to a post, there is a two part entry in Newsfeed. The first part of the entry takes you to the parent post/question and the second part should take you directly to the reply. For the moment, either will take you to the parent post, so you'll need to page down to find the reply you are interested in reading. (I've also reported this bug).
If you are comfortable editing the link address in your browser, you can just delete the end bit of the broken link e.g. '/responses/131379619'. That will take you to the parent post.
Thanks for your patience.
Photo: Antlion about to dig into some sand to create a sand pit trap.