Error: The page must be viewed on a secure channel. CSV download

I added a link in the WordPress admin panel to a script that generates a csv based on a table in the database.
Everything works as expected for me, but my client is getting the following error message:

"The page must be viewed on a secure channel

http error 403.5 - forbidden: ssl is required to view this resource.
Internet Information Services (IIS)
"

The host appears to be running Apache. It has .htaccess files. So why would it be generating an IIS error?

Any insights would be appreciated.

Thank you.
E