the_author_posts() broken after upgrade to 3.1 multi site?

Sorry for my noobish question, I am not a real programmer.
I merged some 2.x blogs to 3.1 and I am using the multi site functionality.
Up until now the following code was working flawlessly but since the upgrade it looks like the variables are empty.

<?php
	if(isset($_GET['author_name'])) :
		$curauth = get_userdatabylogin($author_name);
    else :
		$curauth = get_userdata(intval($author));
	endif;
?>
<?php $pageNumber = (get_query_var('paged')) ? get_query_var('paged') : 1;
$begin = ((($pageNumber -1)*25)+1);
$eind = $pageNumber*25;
$totaal = the_author_posts($curauth->ID);
if ($eind > $totaal) { $eind = $totaal; } 

$naam = $curauth->display_name;

echo "Artikelen $begin t/m $eind (van in totaal $totaal) geschreven door $naam:";

?>

Has anything changed between the two WordPress versions, or does the multi site functionality broke this code?
Any help that points me in the right direction will be appreciated.