List of field items not working on create node - Incorrect integer value: 'item' for column 'delta' at row 1

The creation of a new node fails (with 500 Service unavailable) if there is a field that accepts more than one item, e.g.

<field_some_field>
    <item>First Value</item>
    <item>Second Value</item>
</field_some_field>

The XML structure is the same as retrieved from an example entry of the same ContentType (without the fields that are not writeable).

The following message is the exception that is thrown.

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: 'item' for column 'delta' at row 1: INSERT INTO {field_data_field_is_super_event_of} (entity_type, entity_id, revision_id, bundle, delta, language, field_is_super_event_of_url, field_is_super_event_of_title, field_is_super_event_of_attributes) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8); Array ( [:db_insert_placeholder_0] => node [:db_insert_placeholder_1] => 935 [:db_insert_placeholder_2] => 1306 [:db_insert_placeholder_3] => conferenceevent [:db_insert_placeholder_4] => item [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => h [:db_insert_placeholder_7] => h [:db_insert_placeholder_8] => h ) in field_sql_storage_field_storage_write() (line 448 of /home/www/html/website/modules/field/modules/field_sql_storage/field_sql_storage.module).
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