Results 2 comments of Corebit

Adding the Python-source to the `.spec` file and ```python import multiprocessing multiprocessing.freeze_support() ``` as described by @rokm in https://github.com/pyinstaller/pyinstaller-hooks-contrib/issues/493 did the trick for me!

@T4rk1n , could you apply the try-except logic from https://github.com/plotly/dash/blob/dev/dash/long_callback/managers/__init__.py#L110 to https://github.com/plotly/dash/blob/dev/dash/long_callback/managers/__init__.py#L58 as well? In the current situation, long callbacks still fail for packed applications (eg. Pyinstaller). Fixing this would...