I was able to connect the Harmony client to the EZSrve through 2 different IE8 sessions as well as connecting a java test program I use, all at the same time. I DO NOT SUGGEST TRYING TO RUN multiple Harmony sessions at the same time. What OS, browser, level are you using? Perhaps something in the network security is blocking access to port 8002.
EDIT: how long does it take for the server busy message to display after clicking on Sign In