You can probably fix this by applying some CSS changes to the site, that were posted by @1 some weeks ago. (I don't remember the original thread) Use a chrome extension that can apply CSS changes to websites (I use the extension "Stylebot").
Let it access speedrun.com, and then paste the text in this pastebin: https://pastebin.com/4GyEPMac
Those changes will get rid of the right sidebar while viewing the forums or game leaderboards, and extend the center bar to fit the entire screen.
I understand. Just note that this wasn't meant for fixing any bugs, just returning the forums/leaderboard pages to be similar to how they were before. I just noticed that I can see your forum post just fine with the wide center bar.
You can report that bug in the support hub I guess; none of us can fix that without workarounds like mine.