3D FlipBook - Unexpected server response (403) while retrieving .../test.pdf

Hello good, I write this message to tell you that I have a website where we upload magazines in pdf format using your plugin, but for some strange reason in one of the pdf when opening it shows the following message: message: "Unexpected server response (403) while retrieving PDF" http :: //www.../168.pdf "." Also for some reason sometimes it works and sometimes it doesn't. A greeting !