To the error; if it only randomly happens, it will be hard to track down, but my first though is that it must be something to do with these settings:

https://www.screencast.com/t/EoLysM5EqD

Maybe try changing what they are currently set at?

Assuming you are using the Shortcode Display Results method:

https://searchandfilter.com/documentation/search-results/using-a-shortcode/

If so, I assume you copied the results-infinite-scroll.php template instead of the results.php template when you followed the customising guide there? It still needs to be named results.php in the child theme, so it would need renaming. It looks like this by default:

<?php
/**
 * Search & Filter Pro 
 *
 * Sample Results Template
 * 
 * @package   Search_Filter
 * @author    Ross Morsali
 * @link      http://www.designsandcode.com/
 * @copyright 2015 Designs & Code
 * 
 * Note: these templates are not full page templates, rather 
 * just an encaspulation of the your results loop which should
 * be inserted in to other pages by using a shortcode - think 
 * of it as a template part
 * 
 * This template is an absolute base example showing you what
 * you can do, for more customisation see the WordPress docs 
 * and using template tags - 
 * 
 * http://codex.wordpress.org/Template_Tags
 *
 */

if ( $query->have_posts() )
{
	?>
	
	Found <?php echo $query->found_posts; ?> Results<br />
	<div class='search-filter-results-list'>
	<?php
		while ($query->have_posts())
		{
			$query->the_post();
			
			?>
			<div class='search-filter-result-item'>
				<h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
				
				<p><br /><?php the_excerpt(); ?></p>
				<?php 
					if ( has_post_thumbnail() ) {
						echo '<p>';
						the_post_thumbnail("small");
						echo '</p>';
					}
				?>
				<p><?php the_category(); ?></p>
				<p><?php the_tags(); ?></p>
				<p><small><?php the_date(); ?></small></p>
				
				<hr />
			</div>
			
			<?php
		}
	?>
	</div>
<?php
}
else
{
	?>
	<div class='search-filter-results-list' data-search-filter-action='infinite-scroll-end'>
		<span>End of Results</span>
	</div>
	<?php
}
?>

On line 29 is this:

Found <?php echo $query->found_posts; ?> Results<br />

Simply delete that line and save.

As to the loading icon, this thread might help:

https://support.searchandfilter.com/forums/topic/search-filter-scroll-loading-icon-positionning-in-infinite-scroll/