HtmlViewer Error 403

Good morning Forum,

I wrote a desktop application for online-ordering photos and books.
When finializing the order, I display a web form in a HTMLViewer control.

That is the place where the customer puts in his credit card credentials.

One of my customers has faced the problem that he can’t access the payment web form.
He gets the error message

403 - Forbidden: Access is denied.
You do not have permission to view this directory or
page using the credentials that you supplied.

If he calls Safari or FireFox withe the same URL, the correct web form is displayed-
OS is Catalina - so HtmlViewer renderer doesn’t play a role
Any hints are highly appreciated!

How do you pass credentials to HTMLViewer?

You’re using https right?

@Christian: no
@Greg: yes