Saving field collections

i’d like to update an relation in a field collection - but somehow its not working correctly:

$user = DataObject\User::getById(2)

$fieldcollection = $user->getItemsF(); // get fieldcollection
$currentItem = $fieldcollection->get(0); // get item 0 from fieldcollection

$currentItem->setArray( $array ); // multiselect field -> gets saved - OK
$currentItem->setRelation( DataObject\Stuff::getById(6) ); // many-to-one relation -> won’t get saved

$user->save();

here’s the problem:
the array (multiselect-item) gets saved correctly.
unfortunately the many-to-one relation doesn’t change after saving… can’t null it or replace it with a new object…

any ideas?