WordPress 404 on style.css but not on all pages

By looking at the source of /about-us the culprit seems to be the contact form in the footer, present in more pages of your website.Look here.

Look at the Code module inside the divi builder, you’ll see a <link> tag referring to a style.css: that’s the code that’s causing your 404. By the fact that the link tag is placed before a form I’m getting you wanted to style the form with some custom css? In that case just use the Divi theme options page and paste the css there, giving a proper id or class to the form.