Cannot load media and I get “PHP Warning: Invalid argument supplied for foreach(“

@TheDeadMedic was right, in my second code snippet, admin_init should be admin_menu. That solved my problem, thanks a lot to all of you!

Corrected code:

//remove from menu
function remove_posts_menu() {
    remove_menu_page('edit.php');
    remove_menu_page('edit.php?post_type=page');
    remove_menu_page('edit-comments.php' );
}
add_action('admin_menu', 'remove_posts_menu');