Java - System error

I have compiled successfully compiled my program using javac 1.6.0 and when I submitted I got all test cases wrong with a message "System error".
And I am 100% sure that my program is not problematic.
Is something wrong with the Java compiler or what? I did exit with code 0.

you should read this: http://www.z-trening.com/static.php?page=java_grading

How can I access this page from menu?
I didn't find it in FAQ or somewhere else on the forum when I was looking for it.
Thanks!

Click on some task and in the Submit Solution-part click on How is your code graded?