404 not detected

I’m having trouble with my 404 page, it doesn’t show.
Every URL that i type in is accepted and only leads to an empty page with normal sidebar/menu.
If i go to a page that shouldn’t exist it returns 301 instead of 404 and the 301 points to the same URL as i entered.
If i try to access /index.php?error=404 it just returns the main page.
Does anyone have a clue?

This is the 404.php for my theme:

<?php get_header(); ?>
        <div id="content" class="narrowcolumn">
                <h2 class="center"><?php _e('Error 404 - Not Found', 'minimalism'); ?></h2>
        </div>
<?php get_sidebar(); ?>
<?php get_footer(); ?>

And this is my .htaccess

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

I’m also running WP Super Cache 8.7 but it makes no difference if i disable the plugin.