This problem is thrown by the Classl¡Loader
when it is unable to find the required class.
It can be caused by two problems:
a) The class you're searching for is not inside the JAR file (you can check it uncompressing the jar).
b) You're not using the CLASSPATH you think.
Could you get sure the CLASSPATH the ClassLoader uses is the same you're setting?
In How to get the path of running java program you can find how to get current CLASSPATH of a running JVM instance.