After upgrading from 1.8.6 to 1.9.1 this error comes up on the blog:

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Warning: Cannot modify header information - headers already sent by (output started at /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php:121) in /home/www/blog.website.net/inc/MODEL/skins/_skin.funcs.php on line 71

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

Notice: Only variable references should be returned by reference in /home/www/blog.website.net/inc/MODEL/dataobjects/_dataobjectcache.class.php on line 121

In the ../dataobjectcache.class.php reads from line 111 onward:

/**
	 *  TODO
	 */	
	function & new_obj( $row = NULL )
	{
		$objtype = $this->objtype;
			
		// Instantiate a custom object
		$obj = new $objtype( $row ); // COPY !!
		
		return $obj;
	}

return $obj; is line 121

First question: I know I'm able to silence php errors but I don't know where
Second question: What's happening and what's the solution to this problem?

Thx in advance