Excel cannot open the file claim format.xlsx because the file format or file is not valid. Verify that file has not been corrupted and that the file extension matches the file.
I see this most often when a file has been transferred around a lot and gets corrupted due to bad sectors on hard drives, bad thumb drives, etc…
Occasionally, I find a repair utility that will….kind of….fix it. You tend to lose all formatting and usually the user
Would have to spend more time doing than life than simply starting from scratch. If on a file server, shadow copies (previous versions) are a god send.
If it happens completely randomly, does it happen more often to the same group of people? Perhaps an update malfunctioned?