File format or file extension Not Valid

I ran into this same problem using excel for iPad and iPhone. There seems to be very little helpful information from either Microsoft or Apple.

For me, I believe the issue was file size, which was in excess of 1.3MB. I opened an older copy of the file and stripped out a lot of tabs that were no longer needed, reducing the size to about 850KB. Since doing this I have had no recurring issues and my file that I share across all apple devices is working as one would expect.

I have this same problem, with a twist. It all started after I set up my new Mac Studio. The Excel file performs perfectly fine on my new Mac Studio (M2 Max, 2023). But when I copy it over to my MacBook Pro 13" (2019, i5 CPU) I always get this Error Popup on the MBP:

"Excel cannot open the file 'homeaccess.xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

Both systems are on the same MacOS, latest version (Sonoma, 14.5). The kicker is that this Excel file is password protected in Excel because it holds around 350 different WWW logon passwords. And yes, this Excel file password is known and hasn't ever changed. I can open the file with the password on the Mac Studio. But I always get the same Lock-Out error on the 2019 Intel MBP.

Is this perhaps a Microsoft security "feature" that saves the locked file within the .XLSX file, with an Apple Silicon-specific security key, which does not function properly on the Intel-based MBP? This Seems Plausible.

Another theory is that while both the Mac Studio and the MBP have encrypted file systems running, the MacOS (Apple Silicon) is applying a security key that is not accepted on the MBP's encrypted file system for some reason? (Perhaps because an M2-specific security key is not installed in the MBP because it does not have Apple Silicon???)

If you have any insights into this error (or perhaps just complexity), you would be a Life Saver, due to the nature of the file's contents locking my life out, while I'm on the road away from my home office and my brand new Mac Studio.

To add a further Note: Both the new Mac Studio and the 2019 MacBook Pro have the same, latest version of Microsoft 365 installed (Version 16.86 - 24060916). So it seems unlikely to be a conflict due to differing versions of Excel file formats, etc.

Does the file have a ".xlsx" or ".xls" extension (the end of its file name)? It should. Rename it if you can.

The file always is saved with the same name, ending in "____.xlsx" - Which is the standard Excel file format naming convention for current version Excel files.