Solved
I've solved the problem and I'm going to share the code I used, so other developers that have this issue too can cope with it.
The get_pages()
-function accepts the "hierarchical" argument. By default it's set to 1 (true), which means, that Wordpress searches the pages in hierarchical order to find the other parameters. If - as in my case - you search for pages of an author that are children of pages posted by a different author, Wordpress will not find them!
To avoid this undesired behavior, you have to set the "hierarchical" argument to false (= 0).
This tells Wordpress to search the pages for the given arguments without adhering to the hierarchy.
So to return all the pages of a given author, you have to use the following code:
$pages_args = array(
'sort_order' => 'asc',
'sort_column' => 'post_title',
'authors' => $author_id, // has to be a string!!!
'post_type' => 'page',
'post_status' => 'publish',
'hierarchical' => 0
);