There seems to be a number of issues with the "filesFolder" endpoint and the results are intermittent.
Using Graph to create channels (both private and standard) I then have these errors:
For newly created "private" channels it returns code: "BadGateway" errors. I don't know if they ever go away. The private channel site is created in Sharepoint. The error message: "Failed to execute backend request."
For newly created "standard" channels it always returns code: "NotFound" errors. I can find the channel folder in Sharepoint so I know it exists. The error message: "Sharepoint folder not found."