Project appears to be damaged and cannot be opened, unknown error occurred while saving after update

After automatically updating to the latest verson of Premiere Pro, the App said my project was saved in a previous format and I needed to save it to match the latest version. I did that, but every time I try open the new project file it says "Project appears to be damaged and cannot be opened".

I can't revert to an older version of Premiere Pro to see if it can be opened. I've tried uninstalling and reinstalling ALL Adobe products including Creative Cloud AND removing preferences.

The closest I managed to getting it to work was by importing a select Sequence from the previous project into a newly created project, but then when I go to save it says "Project Save Error: An unknown error occurred while saving the project".

No matter what I do, I hit a deadend. 

can't open my old project in the latest version of the software, and any new projects saved with the new version either will not open, or encounter save errors. I spent months on this project and now I can't do anything. 

Using Windows.