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.