PDF Upload Field Issue

UPDATED:
Okay, so it turns out that I posted an issue like this before but had forgotten. My apologies. <hangs head in geek shame>

The issue appears to be in the PDF creation process. There are hidden elements and un-downsampled mask layers in the construction process. Running the PDF through the Advanced -> Optimize PDF function Acrobat Pro fixes the issue so that the PDF can be uploaded.

However, this is a problem as the client does not own that software and has never had an issue with PDFs on their site before. Any thoughts on how this can be addressed would be welcome!

--- Original Post ---

Hi there,

We have a site in development on a staging server. It has previously allowed us to upload <5Mb .PDF files to an upload field without issue. This is not within the TinyMCE WYSIWYG editor but a distinct field.

As of today, the upload process begins but is fairly quickly stalled. An error message appears in the lightbox of the upload which reads:

Not Acceptable

An appropriate representation of the requested resource /westhaven/cmsAdmin/admin.php could not be found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Any idea where we should begin to find a solution for this?

Many thanks,
J.