That sounds like you did not initialize the freeze support of the multiprocessing module:

https://docs.python.org/3/library/multiprocessing.html#multiprocessing.freeze_support

Basically, you need to add the following code to your main, before anything else:

    from multiprocessing import freeze_support
    freeze_support()

Interestingly, the documentation states that starting a new Process instance without freeze_support will raise a RuntimeError. Maybe the detection of that situation failed in your case?