Hello,
That's strange as our server is not supposed to return 303. If you are able to run packet capture (tcpdump or Wireshark) on the server when this happens and send the result to support@pdfcrowd.com, I can look into it.
If it is a Linux server, you can run tcpdump like this:
sudo /usr/sbin/tcpdump -s 65535 -w /tmp/tcpdump.log