Python environment.
I analyzed the crash with gdb and PyImport_AddModule ("__main__") in Python/import.c:320 seems responsible for the crash: apparently, "__main__" is not found and causes the error "No such file or directory".
I have to figure out what's wrong with the current configuration, especially whether Freestyle's PythonInterpreter can be used as is. I am going to see whether it's just quicker to use Blender's functions.