Just use the code below to figure out which plugin or theme is the culprit. Remember you need to add this to the functions.php file of you active theme

/**
 * Displays hidden plugin and theme updates on update-core screen.
 */
function debug_pending_updates() {
 // Rough safety nets
 if ( ! is_user_logged_in() || ! current_user_can( 'update_plugins' ) || ! current_user_can( 'update_themes' ) ) return;
 $output = "";
 // Check plugins
 $plugin_updates = get_site_transient( 'update_plugins' );
 if ( $plugin_updates && ! empty( $plugin_updates->response ) ) {
 foreach ( $plugin_updates->response as $plugin => $details ) {
 $output .= "<p><strong>Plugin</strong> <u>$plugin</u> is reporting an available update.</p>";
 }
 }
 // Check themes
 wp_update_themes();
 $theme_updates = get_site_transient( 'update_themes' );
 if ( $theme_updates && ! empty( $theme_updates->response ) ) {
 foreach ( $theme_updates->response as $theme => $details ) {
 $output .= "<p><strong>Theme</strong> <u>$theme</u> is reporting an available update.</p>";
 }
 }
 if ( empty( $output ) ) $output = "No pending updates found in the database.";
 echo "<h2>Pending updates</h2>" . $output;
}
add_action( 'core_upgrade_preamble', 'debug_pending_updates' );