I already verified all templates and, apart from "Category", none of them had the "Hide NavBar" option checked. As for cache plugins, I'm not using any.
Regarding refreshing the browser's cache, I did it but it makes little sense: I have the same problem using another computer, another browser or on my smartphone.
I also tried to use the Nav Classic instead of the NavBar, but the problem persists.
Another weird thing is that when I check the page source the navigation code is there, but on "special" pages no <ul> and <li> are present in the <!-- NavBar | Section Template -->:
This is weird, especially if we consider that, when the NavBar is hidden from the Page Options, the code <!-- NavBar | Section Template --> isn't shown at all.