After spending a few days with this issue, I discovered a potential workaround. I added some log.info() lines to the GebRunner.groovy component, and it enabled me to see a NoClassDefFoundError with javax.security.auth.x500.X500Principal. This class is part of Run Time (rt.jar), so it was concerning why it wasn't available. Like you, I put rt.jar in both LOAD_UI_HOME/ext and LOAD_UI_HOME/bundle. Then I looked closer at OSGI, and found LOAD_UI_HOME/conf/config.properties. I found some properties in which I could specify the X500Principal's package (namely, org.osgi.framework.bootdelegation). Once I added the javax.security.auth.x500 package to that line, and restarted LoadUI, I was able to see SSL working! This is only a workaround however, so your mileage may vary.
