XMLRPC Troubleshooting (Timeouts)

I’m having trouble using XML-RPC on our wordpress multisite. Everything works fine using the web interface, so I’m confident it’s not a problem with our upload file size limits or memory settings in php.ini (which I’ve increased in any case). I’ve also enabled xmlrpc and curl in php.ini.

Whenever I try and upload content using XMLRPC (say in the wordpress iOS app or MarsEdit) I get a time out (below is MarsEdit’s network log):

Network message sent: 2013-11-25 20:14:04 +0000
URL: http://glen.blogs.takp.school.nz/xmlrpc.php
Method name: metaWeblog.newMediaObject
Network reply received: 2013-11-25 20:15:34 +0000
URL: http://glen.blogs.takp.school.nz/xmlrpc.php
Method name: metaWeblog.newMediaObject
Status code: -1
Succeeded: NO
–Download Error–
–Timeout Error–

Using xmlrpc.eritreo.it with any of our blogs also returns a 504 error, so I’ve configured something wrong somewhere.

Could you help me debug and fix this? We’d love to use the WordPress iOS app with our students for their blogging.