The Python program:

```
import multiprocessing
import time


class Application:

    def __init__(self):
        self._event = multiprocessing.Event()
        self._processes = [
            multiprocessing.Process(target=self._worker)
            for _ in range(multiprocessing.cpu_count())]

    def _worker(self):
        while not self._event.is_set():
            print(multiprocessing.current_process().name)
            time.sleep(1)

    def start(self):
        for process in self._processes:
            print('starting')
            process.start()

    def stop(self):
        self._event.set()
        for process in self._processes:
            process.join()


if __name__ == '__main__':
    application = Application()
    application.start()
    time.sleep(3)
    application.stop()
```

Its output:

```none
starting
starting
Traceback (most recent call last):
  File "/Users/maggyero/Desktop/application.py", line 31, in <module>
    application.start()
  File "/Users/maggyero/Desktop/application.py", line 21, in start
    process.start()
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/synchronize.py", line 110, in __setstate__
    self._semlock = _multiprocessing.SemLock._rebuild(*state)
FileNotFoundError: [Errno 2] No such file or directory
```

In the function `Application.__init__`, each call `multiprocessing.Process(target=self._worker)` initializes a `multiprocessing.Process` instance with the instance method `self._worker` as its `target` argument. `self._worker` is bound to `self` which has the instance attribute `self._processes`.

In the function `Application.start`, each call `process.start()` serialises the `target` argument and therefore `self._processes`. `self._processes` is a list of `multiprocessing.Process` instances, initially not started yet. The first call `process.start()` starts the first `multiprocessing.Process` instance in that list without issue, but the second call `process.start()` fails.

So a started `multiprocessing.Process` instance cannot be serialised.

The root of the problem is that the `start` method of a `multiprocessing.Process` instance sets its `_popen` instance attribute to a `multiprocessing.popen_*.Popen` instance. The initialization of that instance performs these two steps (among others):

1. For a `multiprocessing.popen_spawn_posix.Popen` instance, a `multiprocessing.popen_spawn_win32.Popen` instance, or a `multiprocessing.popen_forkserver.Popen` instance but not a `multiprocessing.popen_fork.Popen` instance (i.e. for the start method `'spawn'` or the start method `'forkserver'` but not the start method `'fork'`), it [serialises](https://github.com/python/cpython/blob/v3.10.2/Lib/multiprocessing/popen_spawn_posix.py#L47) the `multiprocessing.Process` instance for writing it to the end of the pipe used by the parent process to communicate with the child process so that the child process can execute the `run` method of the `multiprocessing.Process` instance.

2. It [sets](https://github.com/python/cpython/blob/v3.10.2/Lib/multiprocessing/popen_spawn_posix.py#L68) its `finalizer` instance attribute to a `multiprocessing.util.Finalize` instance which itself sets its `_weakref` instance attribute to a `weakref.ref` instance for closing at interpreter exit the ends of the pipes used by the parent process to communicate with the child process. In other words, it makes the `multiprocessing.Process` instance hold a weak reference.

Thus if a `multiprocessing.Process` instance holds a reference to a started `multiprocessing.Process` instance then it holds a weak reference (point 2), so starting it will fail since it will serialise (point 1) the weak reference and weak references are not serialisable:

```
import multiprocessing

if __name__ == '__main__':
    multiprocessing.set_start_method('spawn')  # or 'forkserver' but not 'fork'
    process_a = multiprocessing.Process()
    process_b = multiprocessing.Process()
    process_b.foo = process_a
    process_a.start()  # creates process_a._popen.finalizer._weakref
    process_b.start()  # TypeError: cannot pickle 'weakref' object
```

A minimal Python program showing the serialisation issue:

```
import pickle
import weakref

pickle.dumps(weakref.ref(int))  # TypeError: cannot pickle 'weakref' object
```