You will need to set the return value of the image field to ‘object’, currently it looks set to ‘URL’.
Then you can do something like this:
<?php while ( have_posts() ) : the_post(); ?>
<?php if( get_field('projectafbeelding') ): ?>
<?php while( has_sub_field('projectafbeelding') ):
$image = get_sub_field('afbeelding');
?>
<li style="background-image: url('<?php echo $image['url'];?>');">
<?php echo $image['title'];?>
</li>
<?php endwhile; ?>
<?php endif; ?>
<?php endwhile; ?>
