cannot pickle 'SSLContext' with background callback

Describe your context

Windows 11, Python 3.9

dash                      2.16.1                                                                                               
dash-bootstrap-components 1.4.1                                                                                                
dash-core-components      2.0.0                                                                                                
dash-html-components      2.0.0                                                                                                
dash-table                5.0.0

Occurs with Edge & Chrome.

Describe the bug

I use a background callback in a multi-page app that throws an error when called:

• TypeError: cannot pickle 'SSLContext' object in the browser (without more details, see screenshot below)
• in the standard output:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\201021305\.conda\envs\track\lib\site-packages\multiprocess\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "C:\Users\201021305\.conda\envs\track\lib\site-packages\multiprocess\spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
  File "C:\Users\201021305\.conda\envs\track\lib\site-packages\dill\_dill.py", line 289, in load
    return Unpickler(file, ignore=ignore, **kwds).load()
  File "C:\Users\201021305\.conda\envs\track\lib\site-packages\dill\_dill.py", line 444, in load
    obj = StockUnpickler.load(self)
EOFError: Ran out of input

Here's how the callback is defined:

BACKGROUND_CALLBACK_MANAGER = dash.DiskcacheManager(diskcache.Cache("./cache"))
@dash.callback(
    [...]
    prevent_initial_call=True,
    background=True,
    manager=BACKGROUND_CALLBACK_MANAGER,
)

The callback involves an object that has a SQLAlchemy engine as an attribute. The connection is made through SSL, so I guess this is the object that fails to be pickled. However, I can serialize this object successfully with dill.dumps, so I'm not sure...

Maybe related to https://github.com/uqfoundation/dill/issues/308, but until the issue is fixed, there might be a workaround?

Expected behavior

I expect the callback to run without error.

Screenshots

Start with the background callback definition using dash.callback and DiskcacheManager, then reproduce the Windows 11/Python 3.9 failure with the SQLAlchemy engine involved. Trace where the callback is serialized for the background worker; done means the callback runs without the SSLContext pickling error.