Sorry, I´m opening a new topic because when I try to post in the one that I opened before is saying that is closed.
After such long post and a lot of time trying to figure out a solution for this, I realised that it seems as I was right the first time because it works calling a Simple page with the same URL that a "collection page". The problem was that I didn´t check the box to publish the simple page! It was so easy!!
Thanks for your pacience,
Dani