Thumbnails not showing

Hi there, I’ve got problem with thumbnails. It was working all fine after initial installation, but from 3 posts recently posted it seems not to generate the thumbnails.
have a look here: http://miejskieprzygody.pl/linkedin_goldenline/
WP and plugins are all updated, I’ve run regenerating thumbnails several times as well as posts’ icons are everywhere chosen.
Of what I’ve noticed there is an error in the code, instead of proper url there is a strange number “3001203828”
example:

<li data-position="0" data-poid="in-275" data-post-type="none" >

    <a href="http://miejskieprzygody.pl/dzungla-rynku-pracy/" class="wp_rp_thumbnail">
        <img src="3001203828" alt="Dżungla rynku pracy" width="150" height="150" />
    </a>

    <a href="http://miejskieprzygody.pl/dzungla-rynku-pracy/" class="wp_rp_title">Dżungla rynku pracy</a>
</li>

And here is the correct/working version:

<li data-position="1" data-poid="in-181" data-post-type="none" >

    <a href="http://miejskieprzygody.pl/byredo/" class="wp_rp_thumbnail">
        <img src="http://miejskieprzygody.pl/wp-content/uploads/2014/08/byredo-150x150.jpg" alt="Byredo" width="150" height="150" />
    </a>

    <a href="http://miejskieprzygody.pl/byredo/" class="wp_rp_title">Byredo</a>
</li>

Any ideas what might be the reason??

https://wordpress.org/plugins/wordpress-23-related-posts-plugin/