MS Office 365 Excel can't open ".xlsx" file from calc

The file is a simple list of members names and contact info. plus a count of the number of entries. Is this a bad file, or does MS excel 365 somehow discover that the file comes from something non-Microsoft?

With a simple spreadsheet (no fancy formatting or anything), there normally shouldn’t be any issues. I suspect there’s indeed something wrong with the file itself.

Can LibreOffice still open the .xslx file? If not, that’s a strong sign the file is corrupt.

Otherwise you can try saving it as .ods and opening that in Excel 365.

Failing that, as a last resort, you can try saving as .csv and opening that (note that doing so will preserve data but you would lose formatting).