recv() not ready (11: Resource temporarily unavailable)

Can any nginx / drupal masters shed any light on this?
I've got the upload progress module by apt-get installing nginx-extra. While uploading a file to a site the progress bar doesn't move and my net tab in Chrome shows this response repeatedly:

{"message":"Starting upload...","percentage":-1}

I set the error log to debug and this is what I see while uploading:

2015/03/29 06:54:13 [debug] 11270#0: *4124 http read client request body
2015/03/29 06:54:13 [debug] 11270#0: *4124 recv: fd:119 1460 of 44768
2015/03/29 06:54:13 [debug] 11270#0: *4124 http client request body recv 1460
2015/03/29 06:54:13 [debug] 11270#0: *4124 http client request body rest 2932935
2015/03/29 06:54:13 [debug] 11270#0: *4124 recv: fd:119 -1 of 43308
2015/03/29 06:54:13 [debug] 11270#0: *4124 recv() not ready (11: Resource temporarily unavailable)
2015/03/29 06:54:13 [debug] 11270#0: *4124 http client request body recv -2
2015/03/29 06:54:13 [debug] 11270#0: *4124 http client request body rest 2932935
2015/03/29 06:54:13 [debug] 11270#0: *4124 event timer: 119, old: 1427608513703, new: 1427608513902
2015/03/29 06:54:13 [debug] 11270#0: *4124 post event 0000000002CB74E8
2015/03/29 06:54:13 [debug] 11270#0: *4124 delete posted event 0000000002CB74E8
2015/03/29 06:54:13 [debug] 11270#0: *4124 http run request: "/index.php?q=file/ajax/field_image/und/form-z0RUuKDr9T3PrjXPZA3iks3ZRcA_zR4LzXIrybN4wes"

I'm using Aegir so my config files are split up, but I can post them if it helps. I've added upload_progress uploads 1m; to nginx.conf
I've added the D7 config on this module's page into /var/aegir/config/includes/nginx_vhost_common.conf and added track_uploads uploads 60s; to the index.php location in the same file. That file is called at the end of the vhost template.

Would the error I'm seeing be from my nginx config files or is it likely to be something else?