Error in plugin.php using PHP 5.3

My host has moved my WordPress installations from a server with PHP 5.2 to a server with PHP 5.3. I have 11 installations of WordPress and four of them do not work properly. I get the message:
“Warning: Parameter 1 to ec3_filter_posts_fields() expected to be a reference, value given in …../public_html/stig/wordpress/wp-includes/plugin.php on line 220”

See Stig.Ulfsby.no