Pagination not working it makes no sense

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Here is my code :

<?php get_header();?>

<?php display_breadcrumbs(); ?>

<article>
    <?php if ( is_category() ){
        echo '<h1>' . single_cat_title('', false) . '</h1>';
        echo category_description( );
    }
    else
        echo '<h1><i class="fa fa-tag"></i>&nbsp;' . single_tag_title( '', false ) . '</h1>';
    ?>

    <div class="loop center">
        <?php
        $paged = get_query_var('paged') ? get_query_var('paged') : 1; 

        $args = array(
            'post_type' => 'post',  //Specyfying post type  
            'paged' => $paged       //For pagination
        );

        $loop = new WP_Query( $args );

        if ( have_posts() ) :
            while ( $loop->have_posts() ) : $loop->the_post();
                get_template_part('content');
            endwhile;
            get_template_part('pagination');
        endif; ?>
    </div>

</article>

</section>

<?php 
if( is_category() ){?>
    <div id="dls" class="row">
        <div  class="center">
            <?php
            // Dossier de presse
            $dossier = get_post_meta($post->ID, 'dossier', true); 
            if( !empty( $dossier ) )
                $dossier=" pdf="" . $dossier . '"';
            echo do_shortcode( '[dossier' . $dossier . ']' );
            ?>      
        </div>
    </div>
    <?php
    echo '
    <div id="form" class="row metaform">
        <h2 class="center">Vous souhaitez réaliser un projet similaire&nbsp;?</h2>';
        // echo '<p>/projets/' . get_the_category()[0]->slug . '</p>';
        contact_form( '/projets/' . get_the_category()[0]->slug );
    echo '</div>';
}
?>

<?php get_footer(); ?>

And the pagination.php file :

<nav class="pagination group">
    <?php if ( function_exists('wp_pagenavi') ): ?>
        <?php wp_pagenavi(); ?>
    <?php else: ?>
        <ul class="group">
            <li class="prev left"><?php previous_posts_link(); ?></li>
            <li class="next right"><?php next_posts_link(); ?></li>
        </ul>
    <?php endif; ?>
</nav>

This link does not work :
riehling.mrcoolblog. com/projets/concevoir/

But this one does :
riehling.mrcoolblog. com/concevoir/page/2/

So does this one :
riehling.mrcoolblog. com/concevoir/

I guess there is something wrong with my permalink structure, but I don’t know what it is…

In Yoast, I removed the category base and I added it in the “permalink” settings (as described in the first post), maybe I should remove that and use another way ? What is the clean way to put ‘/projets/’ in front of any archive/post ?