(Possibly OT?) ClassNotFound strange issue

I am not sure if this is because of RW or because I hit some obscure (for me!) Java conundrum. I am facing some Class Not Found exception I cannot explain. It arose in a rather convoluted scenario; this is the simplest case I can arrive at still exhibiting the issue.

Context: 2 plug-ins:
- a Back-End plug-in defining a class (ClassA) to be used by other plug-ins: it is made of two source files: the main plug-in file and the additional ClassA source file
- a Front-End plug-in using ClassA

All tests have been done with a local dedicated server running on the same machine as the client (the client connects via local IP). I have made sure that the Front-End plug-in is loaded before than the Back-End, in case this matters, to be sure to have the most adverse condition (the consumer plug-in is loaded before the provider plug-in is available).

Back-End Plug-in main file (TestBackend.java; minimum possible code!):

package com.vistamaresoft.testbackend;
import net.risingworld.api.Plugin;
public class TestBackend extends Plugin
{
	@Override
	public void onEnable()
	{
	}
	@Override
	public void onDisable()
	{
	}
}

Back-End additional class file (ClassA.java): simply inherits from GuiPanel, adds a field and a simple get/set method pair:

package com.vistamaresoft.testbackend;
import net.risingworld.api.gui.GuiPanel;


public class ClassA extends GuiPanel
{
	private int	value;


	public ClassA()
	{
	}


	public int  getValue()             { return value;     }
	public void setValue(int newValue) { value = newValue; }
}

Front-End Plug-in source file (TestFrontend.java):

package com.vistamaresoft.testfrontend;
import com.vistamaresoft.testbackend.ClassA;
import net.risingworld.api.Plugin;
import net.risingworld.api.events.EventMethod;
import net.risingworld.api.events.Listener;
import net.risingworld.api.events.player.PlayerCommandEvent;
//import net.risingworld.api.gui.GuiPanel;


public class TestFrontend extends Plugin implements Listener
{
	@Override
	public void onEnable()
	{
		registerEventListener(this);
	}
	@Override
	public void onDisable()
	{
		unregisterEventListener(this);
	}
	@EventMethod
	public void onCommand(PlayerCommandEvent event)
	{
		String[] cmd = event.getCommand().split(" ");
		if (cmd[0].equals("/test") )
		{
			ClassA	classA	= new ClassA();
			classA.getValue();
			classA.setValue(1);
//			GuiPanel panel	= new GuiPanel();
//			panel.addChild(classA);
		}
	}
}

(note: commenting out the two last statements is intentional: see below)

With this sources, both plug-ins are loaded correctly and the onCommand method is executed correctly. This means that the definition of com.vistamaresoft.testbackend.ClassA is found and its methods located. So far, so good.

As soon as the two last statements are uncommented (with the respective import at the top), RW terminates at start-up with the error:
java.lang.ClassNotFoundException: com.vistamaresoft.testbackend.ClassA (well before line 31 is executed). The simple presence of a classA parameter in line 31 is enough for something failing in the class loading mechanism.

I tried several cases and the failure always happens when a plug-in contains a statement where an instance of a class defined in another plug-in (another package) is passed as a parameter to a method call. The called method can be a plug-in method or most API methods, there seems to be no difference.

Some API methods seem not affected, for instance event.getPlayer().setAttribute("test", classA); in the above code does not create problems.

Can anyone explain what is happening?

Thanks!

No idea, anyone?