PDF not loading, 403 unexpected server response

I cannot seem to get .pdfs to load into flipbook. Creating an image flipbook works, but loading a pdf stalls giving me this error message.

Please wait! The book is processing…

I have checked for .htaccess problems in the uploads folder, but it is clean. I have ran the “Health and troubleshooting” plugin, but with the same problem of not loading the pdf.

I have exhausted my googling efforts and cannot get the 3d flipbook plugin to load a pdf. I have noticed there are many others with this same problem, but there does not seem to be a clear resolution.

When inspecting the chrome browser console I can see the red alert:

{name: “UnexpectedResponseException”, message: “Unexpected server response (403) while retrieving mysite.com/uploads/pdf (example url is on my site, so no CORS issue.

Other than CORS or .htaccess, what might cause this issue?

Thanks for your time and please help if you know the answer!