I collect data trough Excel document(xls, Excel 2000) and there are like 300 people across the country who fill these excel files and send them to me.

I have an Access 2000 application that reads those 300 xls documents, but it fails to read some of them.
When I open the faulty excel files in Excel 16, Excel 16 displays a red error message bar which states that there might be malicious scripts inside the xls file so it is oppenned in Protected View.

What could be causing this problem? Why the files are marked as malicious?

Can I somehow improve, and somehow handle this sort of error in Access 2000 when it parses them?

Or just use VBA exception handling and print which files are faulty, and then open them manually in Excel 16 and save them as not malicious?