Hi James , I’m having the exact same problem as you .
Glad I found this post .
I just wanna ask if u can explain me better the fix u use :
“In the end it involved going into one of the backup library files and the corrupted library file and replacing one of the event files with the equivalent one from the backup file. More a process of trial and error than anything else but so glad it worked and was pleasantly surprised by Apple’s service.”
You meant you were just copy and paste the event files from one project back up folder to another one until it worked ?
Just trying out to overwrite event file on on the other ?