Rules in Drupal 7: EntityMalformedException in custom PHP
You need to remove the first line $update_node = node_load($node-&gt;nid); because nid does not exist yet in node object then remove the last line as well node_save($update_node);. 

Also, replace each $update_node variable with $node which is available by rules module.

Your final code should look like the following:

$lat1 = floatval($node-&gt;field_geo["und"][0]["lat"]);
//SOME MATH HERE
$node-&gt;field_winkel_dorf["und"][0] = array("value"=&gt;$winkel);