Report Embedded in Iframe Shows "This Content is not Available"

I have a customer in Germany that is having an issue with one of their reports. Our product is a web app that embeds Power BI reports and dashboards of a user's choice inside iframes on a web page, and when this customer goes to display one of their reports, it shows the Power BI loading animation for a few seconds, but then comes up with a message saying "This content is not available. Learn more about Power BI". This happens across Chrome, Firefox, Edge and IE, and checking the console in any of them seems to show no relevant errors. This only happens with a couple of reports as well; the customer seems able to view the rest of their reports/dashboards just fine.

Opening up the report inside the Power BI web page works without issue, even though the customer is using the same credentials to log in in both environments. The same seems to be true for every other user that this report gets shared with as well. With our app, users log in and acquire a token through Azure AD (which has "View All Reports" as parts of its claims, according to the Azure portal). Even when they clear away the token from storage and log in again though, the problem still persists.

The report itself doesn't seem to be doing anything exotic (it just has a standard line graph on it), so I'm having trouble believing that that's the issue. The customer is also using a mix of paid and free Power BI licenses, though users of either license seem to be experiencing the same problem equally. This is also the first customer we've gotten this report from, even though this has been a feature of our product for a couple years at this point. Does anyone have an idea what might be going on here?