Interface casting does not work

Hi,
I am having a problem with an interface:

I have

IAgent is an interface
Agent implements IAgent

var iAgentContainer : IAgent;
iAgentContainer = new Agent();

I use this casting as I want to use only the functions defined in the interface, and I want to inhibit any other public item of Agent.

This works on the computers of 3 of the 4 programmers in our office.
The 4th one receives a casting error:
*[Fault] exception, information=TypeError: Error #1034: Type Coercion failed: cannot convert com.Game::Agent@9250f21 to com.Game.IAgent.

*The new Agent is instanced correctly, but when trying to cast it to IAgent it breaks the balls.
Why?! Why does a PC behave in a different way with the same compiler (FlashDevelop), same SDK, same OS, same folder structure, same everything?