Error when trying to upload documents

I’ve been trying to upload a document via Reports > Documents. In this case, it’s a 768kb pdf file.

I’m met with the error message: "I’ve tried in vain to upload a rollover document via Reports > Documents. I’m repeatedly presented with the error: “Upload of file .pdf failed due to [HTTP_ERROR]: Unsuccessful server response code of: 403”

Anybody know what the problem is, and how to fix?