Unable to open/access project files including auto-save - "sorry, a serious error has occured"

Hello,

I'm hoping someone will be able to help me. I'm running Premiere Pro 2021 latest version on a MacBook Pro running macOS Big Sur. I've been editing a short film - a fairly big 4K project using proxies, and have auto-save turned on.  The only 3rd party app I used is Cinema Grade.I have had no issues until last night when i finished the edit, and tried to export media but the export options box was not coming up. I then saved, closed, and when i tried to open the file again I go this error (see picture), i then tried all my auto-saves and they are doing the same. I also tried to open the file that i saved onto another external harddrive as back up, but that got the same error. Nothing works. I've tried opening a new project and importing from there, but it says it can't be opened. Old projects or very early edits open fine. It seems to be this file.

Please, any advise would be much appreiciated, i've lost so much work and time. 

Thank you!