All categorys shown in custom theme

Hi there

We got a custom wordpress-theme on http://www.sp-entlebuch.ch/, but I can’t reach the developper for the moment, so I need to ask community.
As I mentioned, if you click on either of the category-links (“Aktuell“, “Standpunkte“), posts of any category show u, and not only the ones desired.
If I change theme, categories work fine. If I change the code of the custom theme’s category.php with the one of TwentyFourteen, no more posts are shown. Same if I rename the file.
So either it’s the category.php-file or the loop.php that’s causing the trouble. I don’t know a lot about php, so here’s the code of the two files. Maybe someone finds something.

Category.php:

<?php get_header(); ?>

	<section class="row hr-title">
		<h6 class="small">Alle Beiträge in der Kategorie <?php single_cat_title(); ?></h6>
		<hr class="col-md-12">
	</section>

	<!-- section -->
	<section role="main" class="row trigger" id="container">

		<?php get_template_part('loop'); ?>

		<?php get_template_part('pagination'); ?>

	</section>
	<!-- /section -->

<?php get_sidebar(); ?>

<?php get_footer(); ?>

loop.php

<?php $latest = new WP_Query('showposts=10'); $postCount = 0; ?>

<ul id="blog-list">

<?php
if ( $latest->have_posts()): while ( $latest->have_posts()) :  $latest->the_post(); ?>
	<?php if(has_post_thumbnail()) { $thumb = wp_get_attachment_image_src(get_post_thumbnail_id(), 'medium'); }
	$postCount = $postCount+1;
	?>
	<li>
		<article id="post-<?php the_ID(); ?>" <?php post_class(array('placeholder')); ?>>
			<div class="post-main row">
				<aside class="post-image col-sm-3" style="background-image:url(<?php echo $thumb[0]; ?>);">
					<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">
						<img src="<?php echo get_template_directory_uri(); ?>/img/maske.png" class="img-responsive">
					</a>
				</aside>
				<section class="col-sm-8">
					<h4 class="main-title">
						<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a>
					</h4>
					<p><small>Am <?php the_date( 'j.F.Y' ); ?></small></p>
					<p class="main-content">
						<?php the_excerpt(); ?>
					</p>
				</section>
			</div>
		</article>
	</li>
	<?php if ($latest->current_post +1 !== $latest->post_count) { echo('<hr class="col-sm-8 col-sm-offset-3">'); } ?>

<?php endwhile; ?>

<?php else: ?>

	<!-- article -->
	<article>
		<h2><?php _e( 'Sorry, nothing to display.', 'angelozehr' ); ?></h2>
	</article>
	<!-- /article -->

<?php endif; ?>
</ul>

Thanks in advance.