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movieStim demo not running

dear psychopy

I have tried to run a moviestim.py demo and it is not running

here is what I have done:

in builder menu : View -> Open Coder View
in Coder View menu -> Demos->Stimuli ->MovieStim.py

then I pressed on the green running man and got an error in subprocess popen() failed
subprocess _execute_child failed

when trying to load ffmpeg.

Looks to me as if it is some sort of permissions problem, but I can execute ffmpeg from a command line.

What could be a reason and how to correct this.
I have several installations of psychopy 2 and psychopy3 on different computers
and I tried this on them and got the same result.

So, it looks to me as if there is some sort of installation/configuration problem.

Thanks

Lazar