unauthorized access to reports in a group

I'm having a similar issue - error 401 unauthorized, except that I'm running into it when I list reports in a given group.

I'm able to obtain token successfully using this API:

https://login.windows.net/common/oauth2/token  

- OR- 

https://login.windows.net/{directory_id}/oauth2/token

I'm able to list reports in my personal workspace/group with this API:

https://api.powerbi.com/v1.0/myorg/reports

I'm able to render any of my own reports in an iframe per Power BI REST API documentation.

Now, someone in my organization has shared a few reports with me in another group. I'm able to list all groups I'm shared on using this API:

https://api.powerbi.com/v1.0/myorg/groups

However, as soon as I'm trying to list reports within a particular group (outside my own) I get 401 error using this API:

https://api.powerbi.com/v1.0/myorg/groups/{one_of_the_shared_group_UUIDs}/reports

I'm added to the group with all permissions to view and edit reports in the group and I seem to be able to view reports in the online Power BI portal indeed. I just can't list them via the PowerBI REST API...

Any idea?