Layout bug in Acquia Slate

I have a problem with the layout of the front page and certain of the system pages. A big unwelcome white square appears in the middle of the page, just after the header and before the rest of the content. This big unwelcome white square appears on most but not all pages, and it seems inexplicable. Attached are some screen shots -- two of them display the problem, and the third shows the page as it should appear with no big unwelcome white square. I have spent many hours looking for the cause of this problem, and I am now at the end of my rope. Any help would be appreciated!

PS: I have not edited the style.css file or any of the php files that are part of the slate theme, and i have not created a local.css.