If this also happened to you, the error message "Failed to load data" in Oxygen Builder, Template section, the error comes from a new feature: selecting the view to preview.
If it's empty it shows you an error message.
If you have chosen which post or page to preview, it will work.
Not related to this post :
Intéressés par certains projets cryptos ? One Day soutient l'évolution de l'avenir cryptomonnaie au travers des projets