Can you see where the request comes from? It might be some incompatibility issue, bear in mind RF 3 is pretty old by now.
I thank you for the response. We have identified the component which is firing the invalid request. It's Prototype.js present in the RichFaces.
The following statement fires it:
document.write("<script id=__onDOMContentLoaded defer src=//:><\/script>");
When we commented it; the problem disappears. Now, we are analyzing what should be the best approach to close it.