I have found the problem. I added the following at the end of my main page yesterday. This is when the situation started to happen.
< SCRIPT>
< document.Login.Key.focus()>
< /SCRIPT>
So, by default, even if that history is finished, as soon as my process to load the frames is done, the focus is set back to the main page Login.Key field.
I have disabled it for now. Do you know how I can keep this feature but to avoid this situation?