Output Buffering Issue

I started getting an output buffering conflict on a page that downloads large pdf files. I am getting this php error thrown when I execute a curl session to download the file: <br />
<b>Fatal error</b>: Unknown: Cannot use output buffering in output buffering display handlers<br />

I have excluded the page having the problem, but that didn’t work.

When I deactivate wp-fastest-cache, the problem goes away. The problem page always fails when the plugin is activated.

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