File Format For Numbers Is Invalid

Ever since I updated to Mac OS Catalina 10.15.1. not all, but some of my numbers spreadsheets will have an error message "File Format is Invalid"

Is there anyway to fix this as there are quite a few files. I have back-up on Drop Box and have used it for an important numbers file, with success.

However, this random occurrence is very strange and I wonder if it's prevalent amongst the community?