the link comes from enfold itself. You do not need to insert on title input field links.

because of “Lösungen” now in german

ich sehe deine Seite nicht (als Participant ) aber versuch mal stattdessen:

#top #wrap_all .avia_mega_div {
    background-color: rgba(255,255,255,0.2);
    backdrop-filter: blur(6px)
}

#top #wrap_all #header .avia_mega_div ul, 
#top #wrap_all #header .avia_mega_div ul * {
    background: transparent ;
    color: #000; /**** das war nur um es an meiner Testumgebung umzusetzen ***/
}

die hover styles der menu-punkte bleiben bestehen – was für das navigieren eventuell hilfreich ist.
jedenfalls war es auf meiner Seite so.

well you can insert them by css – just counting the menu-items:
f.e.:
on the left:

#av-custom-submenu-1 .menu-item:nth-child(3) a .avia-menu-text:before {
  content: "\e82b";
  font-family: entypo-fontello;
  display:  inline-block;
  position: relative;
  color: inherit;
  margin-right: 10px
}

on the right:

#av-custom-submenu-1 .menu-item:nth-child(2) a .avia-menu-text:after {
  content: "\e82b";
  font-family: entypo-fontello;
  display:  inline-block;
  position: relative;
  color: inherit;
  margin-left: 10px;
}

in combination with the page id or a custom ID or class on the element is then unique for only that sub-menu

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.

if it is active again –
try:

.html_header_top #top #wrap_all .av_logo_right #header_main .logo {
  position: relative;
  left: 50%;
  right: auto;
  margin-left: unset;
  margin-right: unset;
  transform: translateX(-50%);
  width: auto;
}

you see i have centered the logo a little different – so if you find those margin-left and margin-right values in your code remove them.
i set it to unset to overwrite your settings.

a function like this to remove the image-sizes enfold likes to have:

function remove_enfold_image_sizes() {
// do NOT remove widget size, is used in backend portfolio items!
// remove_image_size('widget');
  remove_image_size('square');
  remove_image_size('featured');
  remove_image_size('featured_large');
  remove_image_size('portfolio');
  remove_image_size('portfolio_small');
  remove_image_size('gallery');
  remove_image_size('magazine');
  remove_image_size('masonry');
  remove_image_size('entry_without_sidebar');
  remove_image_size('entry_with_sidebar');
  remove_image_size('shop_thumbnail');
  remove_image_size('shop_catalog');
  remove_image_size('shop_single'); 
  remove_image_size('shop_gallery_thumbnail');
}
add_action('init', 'remove_enfold_image_sizes');

i guess does only influence the calculation of newly uploaded images.
The former uploads are not influenced on that. So these sizes are still in your uploads folder. (have a look by ftp inspection if this is the fact)
the use of Force Regenerate Thumbnails – removes those already calculated image-sizes that are not registered.

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

i’m participant as you are – so i do not see any private content area.

so you have to wait for mods here

#top .avia-tooltip.avia-tt {
  display: none !important;
}

i’m participant as you are – so i do not see any private content area.

on general layout – Maximum Container Width – this is the value i pull out in the snippet for the setting

$responsive_size = avia_get_option('responsive_size');

If you have there a 100% width set – then it wil be synchronized in the snippet too.
If that is the case – you have to use the other snippet with the “hard-coded” width and replace th 1310px with your 1400px now:

function grid_layout_notfull(){
?>
<script>
(function($){
	$('.av-layout-grid-container.grid-notfull' ).each(function() {
		var notfullID = $(this).attr('id');
		$(this).hasClass('main_color') ? $(this).wrap('<div class="main_color notfullsize '+notfullID+'"></div>') : '';
		$(this).hasClass('alternate_color') ? $(this).wrap( '<div class="alternate_color notfullsize '+notfullID+'"></div>') : '';
	});
	$('.notfullsize').css({"clear": "both", "width": "100%" , "float": "left" , "position": "static" , "min-height": "100px" });
	$('.grid-notfull').css({"max-width": "1400px", "margin": "0 auto" , "padding": "0 50px"});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'grid_layout_notfull');

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

place your contact form inside a 1/1 container.
give a custom ID to that 1/1 container e.g. my-contactform
give a custom class to that 1/1 container mfp-hide
place a button on your site – with manual link : #my-contactform – and custom class: open-popup-link
(because custom class of the button goes to its wrapper the selector .open-popup-link a will work.

btw. in the meantime you do not need this callback .
if you place inside your child-theme functions.php :

add_filter( 'avf_default_lightbox_no_scroll', '__return_true' );

You’re talking about the grid row element, right?

Edit: sadly you did not post a link to your page – but your screenshot is clear enough – i found your site now.
Because it looks exactly the same when you set a grid-row by css to to be not full-width, I thought you had used this element.

____________

So – why don’t you put the portfolio grid inside a 1/1 Container
The Color-Section can have the one background-color – and the 1/1 Container a different one.

if you then do not want on top and bottom of the 1/1 Container the background-color of the Section just remove padding-top and padding-bottom of 50px.

a pure css solution is perhaps not possible.

i have my snippet for child-theme functions.php :

function grid_layout_notfull(){
$responsive_size = avia_get_option('responsive_size');
?>
<script>
(function($){
  $('.av-layout-grid-container.grid-notfull' ).each(function() {
    var notfullID = $(this).attr('id');
    $(this).hasClass('main_color') ? $(this).wrap('<div class="main_color notfullsize '+notfullID+'"></div>') : '';
    $(this).hasClass('alternate_color') ? $(this).wrap( '<div class="alternate_color notfullsize '+notfullID+'"></div>') : '';
  });
  $('.notfullsize').css({"clear": "both", "width": "100%" , "float": "left" , "position": "static" , "min-height": "100px" });
  $('.grid-notfull').css({"max-width": "<?php echo $responsive_size; ?>" , "margin": "0 auto" , "padding": "0 50px"});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'grid_layout_notfull');

what you had to do: give a custom class to the grid-row element: grid-notfull
see: https://webers-testseite.de/grid-row-alb/

the difference between green color-section to following grid-row on my testpage comes from the standard padding of grid-cells of 30px

PS: the ID of the grid-row element goes to its wrapper as class.

Assign a custom class to this gallery – and reduce the value for the maximum width – as on blog pages. e.g: 800px

.avia-gallery.your-custom-class {
  max-width: 800px;
  margin: 0 auto;
}

you can decide to show on grid a smaller version of your images – and in light-box to show the full image. On masonry-gallery you can set to have different column-counts on responsive case.

See: https://webers-testseite.de/condonp/