Hi,
I had the same problem and I used the solution from this post https://kriesi.at/support/topic/critical-error-9/#post-1327504 and replaced enfold\includes\admin\option_tabs\avia_blog.php with the content of https://github.com/KriesiMedia/enfold-library/blob/master/temp_fixes/Enfold_4_8_7_1/includes/avia_blog.php

Think Kriesi should repair this in the next update.

Regards,Ariane

same here, on two different sites critical error and this is the log:
Foutdetails
===========
Een fout van het type E_ERROR werd veroorzaakt op regelnummer 311 van het bestand /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php. Foutmelding: Uncaught ValueError: Unknown format specifier “S” in /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php:311
Stack trace:
#0 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php(311): sprintf()
#1 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/includes/admin/register-admin-options.php(293): include(‘/home/deb45050n…’)
#2 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/framework/php/class-superobject.php(246): include(‘/home/deb45050n…’)
#3 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/framework/php/class-superobject.php(197): avia_superobject->_create_option_arrays()
#4 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/framework/php/class-superobject.php(146): avia_superobject->init()
#5 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/framework/php/class-superobject.php(482): avia_superobject::instance()
#6 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/framework/avia_framework.php(108): AviaSuperobject()
#7 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-content/themes/enfold/functions.php(196): require_once(‘/home/deb45050n…’)
#8 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-settings.php(546): include(‘/home/deb45050n…’)
#9 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-config.php(108): require_once(‘/home/deb45050n…’)
#10 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-load.php(50): require_once(‘/home/deb45050n…’)
#11 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-admin/admin.php(34): require_once(‘/home/deb45050n…’)
#12 /home/deb45050n6/domains/lexdigitalis.nl/public_html/wp-admin/index.php(10): require_once(‘/home/deb45050n…’)
#13 {main}
thrown

Hi Rikard,
We are working with the latest ducth version of wordpress. We updated the enfold thema because of an error in a PHP file,
We instlled the latest version, but still get a critical error.
Now we are working with
WordPress versie 5.8.2
Huidig thema: Enfold (versie 4.8.7.1)
Huidige plugin: (versie )
PHP versie 8.0.13

Foutdetails
===========
Een fout van het type E_ERROR werd veroorzaakt op regelnummer 311 van het bestand /customers/9/0/0/logopedie-oisterwijk.nl/httpd.www/wp/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php. Foutmelding: Uncaught ValueError: Unknown format specifier “S” in /customers/9/0/0/logopedie-oisterwijk.nl/httpd.www/wp/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php:311
Stack trace:
#0 /customers/9/0/0/logopedie-oisterwijk.nl/httpd.www/wp/wp-content/themes/enfold/includes/admin/option_tabs/avia_blog.php(311): sprintf(‘Als u links naa…’, ‘

We see the exact same issue today:

Warning: Undefined array key “footer_option” in /storage/content/50/197550/keymobile.se/public_html/wp-content/themes/enfold/footer.php on line 30 Warning: Undefined array key “footer_behavior” in /storage/content/50/197550/keymobile.se/public_html/wp-content/themes/enfold/footer.php on line 32 Warning: Undefined array key “footer_behavior” in /storage/content/50/197550/keymobile.se/public_html/wp-content/themes/enfold/footer.php on line 270

but if you have that search on your main-navigation the magnifier symbol ( search ) is also on hamburger open visible !
Maybe you only had to style the colors of that Search icon

then you will not need anything else ? express your request a little more precisely.
__________
but if you want it like this: https://webers-web.info
you can hide that custom link in your “non-hamburger” navigation by setting it to display none.
Think of that custom class on that custom link:

#avia-menu li.menu-search {
    display: none;
}

now we got a menu item search – to have the chance to write into that input field – the link on the item itself disturbs – we had to get rid of the href :
this to child-theme functions.php:

function remove_href_from_search_input(){
?>
<script type="text/javascript">
(function($) {
	$('#header').on('click', '.av-main-nav-wrap', function() {
			$('#av-burger-menu-ul .menu-search > a:first-of-type').removeAttr("href");
	});	
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'remove_href_from_search_input');

on my testinstallation i got this in my quick css – but you had to adjust it to your settings and needs:

#avia-menu li.menu-search {
	display: none;
}

#av-burger-menu-ul li.menu-search a:hover {
	background-color: transparent !important;
}

#av-burger-menu-ul li.menu-search a:hover {
	opacity: 1 !important
}

#av-burger-menu-ul li.menu-search .ajax_search_response {
	margin-top: 30px !important;
}

#av-burger-menu-ul li.menu-search .ajax_search_response  * {
	color: #fff;
}

#av-burger-menu-ul li.menu-search .ajax_search_response .av_ajax_search_content .av_ajax_search_title {
	font-size: 16px
}

#av-burger-menu-ul li.menu-search .ajax_search_response  .av_ajax_search_entry:hover .av_ajax_search_title {
	letter-spacing:0.4px;
	color: #004e7f
}

#av-burger-menu-ul li.menu-search .av_ajax_search_image {
	background-color: #fff;
	border-radius: 0;
	color: #000 !important
}