IFRAME embedding with Windows Authentication

This is using Metabase 53.2 but I had the same problem with 52.x.

I have a simple website running on a customer's server to display a list of Crystal Reports the user can view.
It's using IIS and Windows Authentication (HTTP 401 Challenge).
Before the list is displayed, the user is prompted for their user name and password.
I'd like to embed this in Metabase using an IFRAME card so that all their analytics stuff is in one place.
I've added the domain to the list of allowed domains but it's still saying 'There was a problem rendering this content.'
In edit mode, I get:
image

I assume this is something to do with the security.

Any thoughts on a workaround?

can you give us the browser dev tools on the console/network tab?

I thought that Crystal Reports already RIP....

Crystal Reports 2025 is released in 4 weeks or so.New REST API too.
Now it's at SAP, they're just desperate for everyone to buy into their dreadful SAP Analytics Cloud. They've just announced that the SAP BI/Crystal Server products will have another release in 2027, with another pencilled in for 2029. The cynic in me would point out all that lovely recurring support revenue...

There's nothing in the developer console! No related errors.
Could it be due to using a non-standard port?