EntityMalformedException in custom PHP - It used to work

Hi,

I used to run successfully the following custom PHP code before a node is stored

$update_node = node_load($node->nid);
$lat1 = floatval($node->field_geo["und"][0]["lat"]);
//SOME MATH HERE
$update_node->field_winkel_dorf["und"][0] = array("value"=>$winkel);
node_save($update_node);
}

But now I get this error:
EntityMalformedException: Fehlende Paketeigenschaft auf einem Element des Types node. in entity_extract_ids() (Zeile 7663 von /var/www/vhosts/mydomain/httpdocs/includes/common.inc).

I have no idea what changed....

Any help very very welcome.