All I can say is it looks like all the code is working correctly and as far as I'm aware it's working for most other users too. When you see the "Failed to start preview" message, it should log a more detailed error message to the console. You showed a picture of the console, but it did not include this message, because the message starts with "Error launching preview: ". If you can find that message in the console and share it, it may shed some light on this. Other than that I can only guess, and I think the most likely issue is that there was a network problem downloading C3 for offline use so it's not actually fully cached. You could try clearing your browser cache and retry loading it online and making sure you wait for the "ready to use offline" notification.