The API call does not hit our server at all. It looks like it is blocked by a firewall or there is another restriction issue which prevents the client library from connecting to our server. Were there any changes to your network configuration that occurred recently?
The client library needs to send a POST request to pdfcrowd.com, ports 80 (http) and/or 443 (https). I would recommend that you contact your ISP or network administrator to see if they're able to troubleshoot the problem.
If your application runs on Linux, could you send us the output of the following command?
curl -v -F username=username -F key=apikey -F src=1 http://pdfcrowd.com/api/pdf/convert/html/ > /dev/null