I solved the issue by changing protocol=None in pythons multiprocessing/reduction.py to 4.
class ForkingPickler(pickle.Pickler):
    '''Pickler subclass used by multiprocessing.'''
    _extra_reducers = {}
    _copyreg_dispatch_table = copyreg.dispatch_table

    def __init__(self, *args):
        super().__init__(*args)
        self.dispatch_table = self._copyreg_dispatch_table.copy()
        self.dispatch_table.update(self._extra_reducers)

    @classmethod
    def register(cls, type, reduce):
        '''Register a reduce function for a type.'''
        cls._extra_reducers[type] = reduce

    @classmethod
    def dumps(cls, obj, protocol=4):
        buf = io.BytesIO()
        cls(buf, protocol).dump(obj)
        return buf.getbuffer()

    loads = pickle.loads

register = ForkingPickler.register

def dump(obj, file, protocol=4):
    '''Replacement for pickle.dump() using ForkingPickler.'''
    ForkingPickler(file, protocol).dump(obj)

Thank you for your help!