the project could not be loaded, it may be damaged or contain outdated elements

I am trying to create a new premiere pro project that combines 5 old projects. I made the projects a year ago, but have been updating them in the past week: they all run fine on the latest Premiere Pro 14.3.1 and have been saved with a new name. I have tried importing the projects into a new project, but I get the error message: "the project could not be loaded, it may be damaged or contain outdated elements"

Any way around this? 

How can I diagnose the problem if each of the 5 individual projects open fine?

Thank you!