Next/Previous Post within same Category

try first this shorter form – maybe that will work for you.
(do not know if the old filter is still ther avia_ …)
there are more complex snippets – but in almost all installations this will work:

function enfold_customization_postnav($settings){
  $settings['skip_output'] = false;
  $settings['same_category'] = true;
  return $settings;
}
add_filter('avf_post_nav_settings','enfold_customization_postnav', 10, 1);

edit: here is one with more options:

function my_avf_post_nav_settings( array $settings ){
  if( true === $settings['is_hierarchical'] ){
    $settings['skip_output'] = true;
    return $settings;
  } 
  if( ! in_array( $settings['type'], array( 'post', 'portfolio' ) ) ){
    $settings['skip_output'] = true;
    return $settings;
  }
  $settings['same_category'] = true;
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false; 
  return $settings;
}
add_filter( 'avf_post_nav_settings', 'my_avf_post_nav_settings', 10, 1 );

btw.: https://github.com/KriesiMedia/enfold-library/blob/master/actions%20and%20filters/Layout/avf_post_nav_settings.php

with this snippet you do not even need to set the enfold options on post-navigation.
You can set with that filter even to loop:

function my_avf_post_nav_settings( array $settings ){
  if( true === $settings['is_hierarchical'] ){
    $settings['skip_output'] = true;
    return $settings;
  } 
  // the post-types inside the array are allowed to have post-navigation
  if( ! in_array( $settings['type'], array( 'post', 'portfolio' ) ) ){ 
    $settings['skip_output'] = true;
    return $settings;
  }
  $settings['loop_post_nav'] = true;
  $settings['same_category'] = true;
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false; 
  return $settings;
}
add_filter( 'avf_post_nav_settings', 'my_avf_post_nav_settings', 10, 1 );

btw. if you set this (is_hierarchical) condition to false – then even pages will have post-navigation

btw: where does the p-tag come from : p.ondertitel
See DOM:

you can see that if there are more than 1 post inside a category – it works:
https://kunst-en-verhalen.rhijnhof.nl/pieta/

what makes me wonder is that on your posts with category: oorlogsslachtoffers (only 1) – there should be no post-navigation. ( Do you have in the page title that p-tag ?)
so maybe Ismael could help you with this inside your installation

PS : the setting on Enfold > Blog Layout > Single Post Navigation setting is not important if you use the filter from : above

With the help of this filter, you can even handle post-types differently.