Just to stop the ball of comments not related to my case:
As I said before, the site is not mine.
It doesn't allow robots somehow, because I can browse the pages but sphider can't get them.
The site is OLD, custom made. It's not wordpress nor uses plugins.
If it does, I don't have access to it to edit a thing.
Let's start from the fact that the site is already as I said, no way to change it, and I'll access it from an external server.
It might have a "search" feature, but I don't know all the common query strings/variables commonly used by old forum platforms for me to test it. (again, I think it was completely custom made, but you know, there has always been trends in programming)
Can you tell?
The urls end with (e.g.) /org_board.show_msg?an_msg_id=1787112
and the main boards page is at /org_board.p_main
Can you guess the search query?