Error with customized single.php since last update (1.4.2.1)

Since the last update, all my posts were white. Only the title, no texts, no pictures !?
Category pages were ok, only the posts were affected.

I searched and localized the problem : the child single.php file.
I delete it from child theme. All is ok.
But how to keep the customized functions I used to have with my child single.php ?

The functions are :
1- next and previous buttons at bottom of the post page go to next/previous post of the same category (instead of any category)
2- a back button to the category page

The code I used to have for that:

 %title', TRUE ); ?>
', TRUE ); ?>
					<?php
					$myLstCat = (array) get_the_category(intval(get_the_ID()));
					if (count($myLstCat) >= 1) {
					$myCat = &$myLstCat[0];
					$myCatLabel = $myCat->cat_name;
					$myCatLink = get_category_link(intval($myCat->cat_ID));
					?>
">Retour vers 

<?php
}
?>
<!-- #nav-below -->

Do I have to change this code ? Add new one in another php file, … ?

Website: www.lysithee.com