To further assist you with this issue, could you please confirm if this happened with all files you created with the Whiteboard app or with some of them? Did they contain specific objects or activities?
Are you able to open these files from the main menu of the Whiteboard app?
Please, try to clear the cache and data from the Whiteboard app, from Settings, Apps, Whiteboard, Storage, Clear cache and data.
What model of ActivPanel and computing device do you have? Is the software up-to-date?