Comments on individual posts are throwing off the formatting

Hello,

I built my site years ago (Twenty Eleven theme) and don’t update it regularly. I noticed recently that the formatting was suddenly weird on individual posts (home page is fine) — on certain posts, the white background color disappeared, and the right sidebar was missing, and comments weren’t showing up. Then I realized it’s ONLY happening on posts that have comments. Any pages with no comments look normal.

Any idea what I can do to fix this? Is there something wrong with the comments part of my stylesheet?

Apologies for not explaining this better – I worked very hard on this site years ago without a tech background, and it’s largely remained unchanged since. I really want to fix this though, as I have lots of posts up and want to keep my site working.

Thanks for any help.
—Erica

The page I need help with: [log in to see the link]