graphene relay "Resolved value from the connection field have to be iterable or instance of"

Question

I want to use graphene with relay together with a custom database connector. My schema is looking like:

class DBItem(DjangoObjectType):
    class Meta:
        model = DB

        filter_fields = {
            'id': ['exact'], 
            'LOGID': ['exact']
        }

        interfaces = (relay.Node, )

class DBConnection(relay.Connection):
    class Meta:
        node = DBItem

When I use only grahene my resolver works good when I build a list of the objects:

class Query(graphene.ObjectType):
    sf = graphene.List(DBItem)

    def resolve_sf(self, info):
        db = MyConnector()
        items = db.read_items()

        db_items_as_obj_list = []

        for item in items:
            i = DBItem.__new__(DBItem)
            i.__dict__.update(item)
            db_items_as_obj_list.append(i)

        return db_items_as_obj_list

What I understood is, that in order to use relay, I have to define a relay Connection, which I did, otherwise I will get a "table not found" error. The MyConnector().read_items() returns a dictionary. But what I dont understand is which type of object shall the resolver return.

categories = relay.ConnectionField(DBConnection)

I'm always getting:

graphql.error.located_error.GraphQLLocatedError: Resolved value from the connection field have to be iterable or instance of DBConnection. Received "None"