I have a theory on what's going wrong for you. When RobotBuilder starts up, it tries to load the robot configuration file it was working on previously. The name of the file it was most recently working on is stashed using Java's Preferences utility. I think that on your system it is mixed up, and is reading a file that is isn't a valid RobotBuilder YAML file.
If we clean out RobotBuilder's preferences, then it should revert to opening up a blank robot configuration.
Unfortunately, I don't know that there is anything in the RobotBuilder code to self-correct on this subject. Java preferences are stored in different locations for each different operating system. Macs keep them in PLIST files in the user Library. I think Windows stashes them in the registry. Linux has its own set of files.
The simplest solution I can think of is to run a short Java program that cleans out the RobotBuilder preferences. Try compiling and executing the following. It won't do any harm to your system:
Code:
import java.util.prefs.Preferences;
public class PreferencesCleaner {
public static void main(String[] args) throws Exception {
Preferences prefs = Preferences.userRoot().node("robotbuilder.MainFrame");
prefs.clear();
}
}