Desktop Excel is 2003, Excel for the iPhone, OneDrive

Error when attempting to load a spreadsheet from OneDrive with Excel on the iPhone: “This workbook can’t connect to the external data or it contains macros that will not run in this version of Excel”

I tried 3 random spreadsheets, all basic in composition.

The only macro involved is one that I was given here that adds the date the file was last edited in the footer.

I have no idea what “can’t connect to the external data” means?

Since Apple's 'Numbers' app has no problem loading these files, what’s the deal with Excel for the iPhone?

Mark