If anyone's still having problems with connecting, I had Mindcrack all set up correctly on both Raeka's computer and my own, however it was getting as far as the "Logging In" then after hanging for a while after the messages about Portal Gun errors, simply shutting down.
Last night I tried reinstalling the latest version of Java on Raeka's machine (when reinstalling over the top, it says "This version is already installed. Do you wish to reinstall it?" at which stage it goes through the motions of uninstalling and reinstalling Java) but that didn't fix it.
However tonight I tried completely uninstalling Java, and installing it again fresh. This fixed this particular error.
So, if it's all set up correctly, and the server isn't throwing any incorrect mod errors, and you're getting all the way to handshaking with the server and then it's unexectedly closing - try uninstalling and reinstalling a fresh copy of Java.