Http connector oauth empty response no error

[Alesch]

Dear community,

I try to query the following API for reservation information: 

https://management.azure.com/providers/Microsoft.Billing/billingAccounts/XXXXXXX/reservations?api-ve...

We created an App Registration and according Enterprise App with the right level of permission. The authorisation seems fine, but unfortunate I got only an empty array as an answer. When testing the API with my user account (has the same role assigned) I get the expected response.

Any ideas to debug this?