This project is invalid or corrupt...

This is a very hard lesson to have to learn about the poor quality of how Storyline handles its file management.  I don't know why Articulate can't/don't make this aspect of Storyline performance more reliable as it seems it has always been this way.  Just look through the forums for the reports of file corruption.

My workflow now is to make regular (sometimes 2 or 3 per day) incremental file saves.  I add a number to the end of the file name and save a new .story file every time I complete a new scene or create something complex within a scene.  At the end of the project I might end up with somewhere between 50-100 storyline (.story) files for a project.  I think my record was around 135.  Fortunately disk space is reasonably affordable.