We all hope that this page is rarely needed, but when it is displayed, I noticed that the colourful category boxes are shown after each topic title under “Popular” and “Recent” and that means that the box sometimes ends up on a line by itself making the visual impression a bit confusing:
Suggestion: Put those little boxes before each title instead of after.
And while I’m at it: My impression/guess is that the page is often displayed when people access closed categories on a new device (e.g. via email link). To less tech-savvy users, this can be confusing, especially when they are not used to thinking about logging in (thanks to the long default login time). So I changed the copy for the message on that 404 page to
Oops! That page doesn’t exist or is private. Are you logged in?
Maybe that would be something to consider as the default copy?
Just a suggestion.