Both works fine here in Java 8, so maybe restarting the rumble/cleaning the caches helps? btw I would like to run Gigarumble as well, may you post a working config file for it? Hope that is included in the next release of robocode.
I removed the caches from all installations (RoboRumble and GigaRumble are separate installs), and now XanderCat also works! I can't run battles till I get home though, as the firewall at work prevents download/upload of necessary files and results.
My config file is listed in the discussion below. The gigarumble.bat file contains:
java -Xmx512M -cp libs/robocode.jar;libs/roborumble.jar;libs/codesize-1.1.jar; roborumble.RoboRumbleAtHome ./roborumble/gigarumble.txt