StanModel' object has no attribute 'fit_class'

I have tried everything to solve this issue - all of the references I can find online are for older versions of prophet, and are a few years old:
I am trying to install using Conda on Windows 10 which the installation instructions suggested was the best way - the first attempt failed until I installed GCC (version 5.3.0) - then it installed.
Windows Version is 10.0.19043
I meet all of the requirements here - https://github.com/facebook/prophet/blob/master/python/requirements.txt
I have used the following line to install:
`conda install -c conda-forge prophet`

And I still get the following error - this seemed quite common a few years ago but I cannot find any recent comments about it:

`WARNING:pystan:No module named 'stanfit4anon_model_dfdaf2b8ece8a02eb11f050ec701c0ec_4959769199373391054'
WARNING:pystan:Something went wrong while unpickling the StanModel. Consider recompiling.
INFO:prophet:Disabling daily seasonality. Run prophet with daily_seasonality=True to override this.
Traceback (most recent call last):
File "C:\Users\iwatt\Coding\prophet\main.py", line 12, in
m.fit(df) # Fit the Prophet model.
File "C:\Users\iwatt\Coding\prophet\env\lib\site-packages\prophet\forecaster.py", line 1171, in fit
self.params = self.stan_backend.fit(stan_init, dat, **kwargs)
File "C:\Users\iwatt\Coding\prophet\env\lib\site-packages\prophet\models.py", line 264, in fit
self.stan_fit = self.model.optimizing(**args)
File "C:\Users\iwatt\Coding\prophet\env\lib\site-packages\pystan\model.py", line 542, in optimizing
fit = self.fit_class(data, seed)
AttributeError: 'StanModel' object has no attribute 'fit_class'`

Can anyone help?