We were looking for a stronghold with eyes of ender, but when we arrived at the place where the eye of ender led us, there was NO stronghold. We intensivly searched the entire ground but we are 100% sure that there is no stronghold.

As far as I know there is no way to reproduce it. (generate structures was ON)When I generated a new world and used enderpearls to find a stronghold, it was there like it should be.

As you can see on the screenshot all the eyes of ender float there, which suggests that there should be at least an End portal.