manim: The system cannot find the file specified when rendering

Question

I have looked for days on the net, and no answer has helped.

I run a manim script and I get this message at the end:

FileNotFoundError: [WinError 2] The system cannot find the file specified:
'C:\\_D_Drive\\_PROGRAMMING_JP\\python\\chryzoid_base\\media\\videos\\cs_alone\\3000p30\\CS_alone.mp4'

I can see that the temporary mp4 files are there, in

C:\_D_Drive\_PROGRAMMING_JP\python\chryzoid_base\media\videos\cs_alone\3000p30\partial_movie_files\CS

The worst of it is that the problem can occur with some classes in the same .py file and not others.

Any pointers...

And of course, I can provide all the (ugly) code I could be asked for...