NoClassDefFoundError in max for live

I'm working on a max for live device for a client. On my computer everything works fine, but on the clients computer I get a runtime error:
It inits the object but when a method is called I get: NoClassDefFoundError.

import com.cycling74.max.*;
import com.cycling74.msp.*;

public class BufferTool extends MaxObject {

    String buffer = null;
    String Temp = null;

    BufferTool(Atom[] buff) {
        declareInlets(new int[] { DataTypes.ALL });
        declareOutlets(new int[] { DataTypes.ALL });
        createInfoOutlet(false);

        if (buff.length > 0)
            set(buff[0].toString());
    }

    public void set(String name) {
        buffer = name;
    }

    public void setTemp(String Temp) {
        this.Temp = Temp;
    }

    public void combine() {

        MaxSystem.deferLow(new Executable() {
            @Override
            public void execute() {

                ...

            }
        });
    }

}

The java.class file is in the max search path and the max.java.config has been edited.

Any help will be much appreciated!!