Python Errno 9 Bad file descriptor in Mac OS X

I have the following code running without any problem multiple times after each other in Linux:

def test_ftp(ftpserver):
    with FTP() as f:
        f.connect("localhost", port=ftpserver.server_port)
        f.login("M1", "bachmann")
        f.cwd("/")
        f.mkd("FOO")
        f.quit()

The same tests can only be run once in MacOS X, after that it will simply hang. Rebooting the machines, is the only way I can re-run the tests.

ftpserver is a test fixture defined in pytest-localftpserver, I am posting the code for this fixture here for the reason that I suspect that it is the cause of error:

class MPFTPServer(multiprocessing.Process):

    def __init__(self, username, password, ftp_home, ftp_port, **kwargs):
        self._server = SimpleFTPServer(username, password, ftp_home, ftp_port)
        self.server_home = self._server.ftp_home
        self.anon_root = self._server.anon_root
        self.server_port = self._server.ftp_port

        super().__init__(**kwargs)

    def run(self):
        self._server.serve_forever()

    def join(self):
        self._server.stop()

    def stop(self):
        self._server.stop()

@pytest.fixture(scope="session", autouse=True)
def ftpserver(request):
    """The returned ``ftpsever`` provides a threaded instance of
    ``pyftpdlib.servers.FTPServer`` running on localhost.
    ...
    """

    from pytest_localftpserver.plugin import MPFTPServer
    ftp_user = os.getenv("FTP_USER", "fakeusername")
    ftp_password = os.getenv("FTP_PASS", "qweqwe")
    ftp_home =  os.getenv("FTP_HOME", "")
    ftp_port = int(os.getenv("FTP_PORT", 0))
    server = MPFTPServer(ftp_user, ftp_password, ftp_home, ftp_port)
    # This is a must in order to clear used sockets
    server.daemon = True
    server.start()
    yield server
    server.join()

Can you tell why this code "works repeatedly" in Linux but not in MacOSX?

update

Digging a bit further, I found that the ftp server will not even start, hence the hanging. The code crashes with the following message:

    Process MPFTPServer-1:
    Traceback (most recent call last):
      File "/opt/pkg/lib/python3.5/multiprocessing/process.py", line 249, in _bootstrap
        self.run()
      File "/Users/w/.virtualenvs/controller_config/lib/python3.5/site-packages/pytest_localftpserver/plugin.py", line 81, in run
        self._server.serve_forever()
      File "/Users/w/.virtualenvs/controller_config/lib/python3.5/site-packages/pyftpdlib/servers.py", line 207, in serve_forever
        self.ioloop.loop(timeout, blocking)
      File "/Users/w/.virtualenvs/controller_config/lib/python3.5/site-packages/pyftpdlib/ioloop.py", line 348, in loop
        poll(soonest_timeout)
      File "/Users/w/.virtualenvs/controller_config/lib/python3.5/site-packages/pyftpdlib/ioloop.py", line 709, in poll
        timeout)
    OSError: [Errno 9] Bad file descriptor