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.