Hi,
yeah, that method set_value_serialized_string is kinda misleading in this case. The output directory is of type unreal.DirectoryPath() and you need to set this type in serialized form.
In our scripts we’re setting the path using unreal.DirectoryPath('../outputdirectory').export_text() and that works for us.
your last line should be
job_override_variables.set_value_serialized_string(OutputDirectory_graph_variable, unreal.DirectoryPath('C:').export_text())
that should hopefully work for you.
Cheers