"Content not available" error when integration Power BI report from user group

Hi all, I am integrating Power BI report in my own web app and need some help on this error.

1. My reports published to user groups in app.powerbi.com
2. I have granted access of my end users to this user group.
3. I am loading these reports dynamically in my custom web application using Power BI JavaScript SDK.
4. My server side code is accessing this endpoint to retrieve my report access token and other details https://api.powerbi.com/beta/myorg/groups/{group id}/reports 

However, the report cannot be loaded. After user sigin their Power Bi account and grant acess to my web application, the Power BI splash screen appears. A few seconds later, it displays a "Content not available" error.

Note that I can retrieve report name, embedded url, access token etc. from Power BI API.

But if my user also sign in to app.powerbi.com in a diferent browser tab, the my web application can load the report correctly.

Why do they see the error message?