This morning I noticed the attached image when attempting to start the Intel XDK. It appears McAfee update itself and is now blocking the XDK from connecting to 127.0.0.1. We are currently trying to debug this. In the mean time, you can manually allow the XDK through the firewall by adding the nw.exe to the allowed processes.
If you installed the XDK to the default location that would be:
I will update this post when I know more.