I had this problem and it was caused by the variables.less file. Overriding it to set the icon-font-path value solved the problem.
The file structured looks like this:
\Content
\Bootstrap
\Fonts
styles.less
variables.less
Adding my own variables.less file in the root of Content and referencing this in styles.less resolved the 404 error.
Variables.less contains:
@icon-font-path: "fonts/";