Wordpress: authors page with custom query - wordpress

Question

On a wordpress site I am coding an authors page. Therefore I use a custom query that I also use in several other parts of the site. Partly it works fine, but as it has a major bug, I cannot use it:
I receive not only a chosen autors post. All posts of all authors are listet.
If I use the main loop, I do not have that problem. But because of the pagination which should be the same everywhere, I would like to use the custom loop. Where is my mistake?
Here my code:
<?php
$curauth = (isset($_GET['author_name'])) ? get_user_by('slug', $author_name) : get_userdata(intval($author));
?>
<section class="tk_fullwidth">
<div id="primary" class="content-area">
<h1 class="page-title">Beiträge von: <?php echo $curauth->nickname; ?></h1>
<div id="main" class="site-main" role="main">
<div class="grid" data-masonry='{ "itemSelector": ".grid-item", "columnWidth": 285, "gutter": 20 }'>
<?php
$paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1;
$query = new WP_Query( array(
'post_type' => 'post',
'post_status' => 'publish',
'posts_per_page' => '50',
'paged' => $paged,
'order' => 'DESC',
'orderby' => 'date',
) );
?>
<!-- begin loop -->
<?php if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post(); ?>
<div class="grid-item">
<a class="xyz" href="<?php the_permalink(); ?>"></a>
<h3 class="entry-title"><?php the_title(); ?></h3>
<?php
if ( has_post_thumbnail() ) {
the_post_thumbnail('medium');
}
?>
</div><!-- grid-item -->
<?php endwhile; ?>
<!-- end loop -->
<div class="tk_pagination">
<?php
echo paginate_links( array(
'base' => str_replace( 999999999, '%#%', esc_url( get_pagenum_link( 999999999 ) ) ),
'total' => $query->max_num_pages,
'current' => max( 1, get_query_var( 'paged' ) ),
'format' => '?paged=%#%',
'show_all' => false,
'type' => 'plain',
'end_size' => 2,
'mid_size' => 1,
'prev_next' => true,
'prev_text' => sprintf( '<i></i> %1$s', __( '<', 'text-domain' ) ),
'next_text' => sprintf( '%1$s <i></i>', __( '>', 'text-domain' ) ),
'add_args' => false,
'add_fragment' => '',
) );
?>
</div>
<?php wp_reset_postdata(); ?>
<?php else : ?>
<p><?php _e( 'Sorry, no posts matched your criteria.' ); ?></p>
<?php endif; ?>
</div>
</div><!-- #main -->
</div><!-- #primary -->
</section><!--tk_fullwidth-->