It had to be just the right kind of Java…

More fun with JabRef and the AWT toolkit. Starting JabRef by java -jar JabRef-2.4.2.jar got me a string of errors and no GUI:
ERROR while starting or running JabRef:
java.lang.NoClassDefFoundError: Could not initialize class java.awt.Toolkit
The reason for this appears to be that openSuSE uses the non-proprietary OpenJDK Java runtime by default, while you need Sun's JRE for JabRef to work (available in the non-oss repo).
Check with java -version - if it says "OpenJDK" instead of "Java(TM)", you need to switch the packages in YaST or zypper.