load a different stylesheet in a category post

You can do it like-

function wpse_enqueue_post_template_styles() {
    if ( is_category('bedrijven') ) {
        wp_enqueue_style( 'post-template', get_stylesheet_directory_uri() . '/layout-interieur.css' );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse_enqueue_post_template_styles' );

Here bedrijven is your category slug.

As you said the above method hasn’t worked for you, so here I’m suggesting another method-

First remove the first method’s full code. Then follow this below method step by step-

Step 1:
In your theme’s functions.php put the below code block-

function the_dramatist_register_category_styles() {
    wp_register_style( 'post-template', get_stylesheet_directory_uri() . '/layout-interieur.css' );
}
add_action( 'wp_enqueue_scripts', 'the_dramatist_register_category_styles' );

By this above code block we are registering your stylesheet file at WordPress. So we can call it anytime.

Step 2:
Now go to your category-bedrijven.php file and inside this file put the below code-

<?php wp_enqueue_style('post-template'); ?>

It’ll must work if you do it right.

Hope that helps.