I had this problem when behind a firewall.

I got around it by editing:

/Applications/Xcode.app/Contents/Applications/Application\ Loader.app/Contents/MacOS/itms/java/lib/net.properties 

If you do not find net.properties in above mentioned path then try, as it is changed now.

/Applications/Xcode.app/Contents/Applications/Application\ Loader.app/Contents/itms/java/lib/net.properties 

Change the line:

# https.proxyPort=443

To:

https.proxyPort=80

This enables the application to use the HTTP port for HTTPS. Worked straight away after this. Hope this helps.

To be honest, this may not be an answer but it really worked for me.

1. I launched Application Loader and started submitting.While sumbitting was going on,
2. I launched Organiser and select Distribution over archiver in Xcodeas well and started uploading concurrent.

They worked together and 2nd one completed the submission.

Just my experience.

I tried many solutions like using port 80 for https connection but none of the solution worked for me.

https.proxyPort=443

tohttps.proxyPort=80

Whenever I face this issue, I upload application from two different instances of application loader and it works like a charm.

I have two different versions of application loader on my machine, one came with XCode4 and one came with XCode5. It always works if I upload application from two instances simultaneously.