The Loop: article leaking outside of div.section.main

I have a category template here: http://referendum-autoceste.hr/category/vijesti/ If you look at the source, first 4 <article> elements are inside div#content section#primary main#main, which is excellent. But then, the rest of the <article> elements show up *after* </main></section></div>. I looked into archive.php (that’s where category Loop is defined), and everything looks as expected: The Loop (<?php while ( have_posts() ) : the_post(); ?>) is inside <div><section><main>. But, when the page is rendered, first 4 <article> elements are inside those tags, and next 6 <article> elements are outside. That makes absolutely no sense.

Moreover, if you click on “older posts” and go to page http://referendum-autoceste.hr/category/vijesti/page/2/, then everything is just fine! All 10 <article> elements are inside <div><section><main> and everything looks fine.

I’m completely confused by this. How come the template works fine on page 2, but not on page 1? How do the <article> elements end up outside their parent tags? That just shouldn’t be possible. Has anybody every seen anything like this?

Kind regards,
Nikola