This project is invalid or corrupt......

Ok,  

This is not really going to help recover your material but this seems to be a fix for it happening again in Storyline2 (not sure about Storyline).  Well it is for me anyway.   I was creating content on one machine and then transferring to a laptop.  Each time I did this (it did not matter on which machine I created the content) I would try to open the file on the other machine and I would get the "Corrupt" error message.  

I was transferring between machines via the cloud.  I started to check with other colleagues and sometimes they could open the files and sometimes they could not.  

What I have discovered is that I can save the storyline file anywhere on either machine, then whenever I want I can upload them to the cloud.   However if I did this and the story file was still open in Storyline2 the file would then be corrupt.  If I try to move the file to a USB key I get a message saying that the Storyline file is still open and so this task cannot be completed.  I don't get this message when I am uploading into Google Drive and so have been inadvertently uploading material while the story file was still in use.  Google drive even puts a different icon on the storyline file if you upload while its still open in Storyline.

Hope this helps someone.