@Alessio Vietri
Thank you for your time and patience throughout this issue! I received a response from our engineering team and it looks like this could potentially be related to your O365 License.
Update:
- Can you make sure the tenant has a valid SharePoint online subscription. Looks like the test user does not have an O365 license. Can you please verify?
- Need clarification from customer on what exactly was working on Graph Explorer - Graph Explorer uses User token, but it looks like the customer is using client credential flow to get the token. Client credential flow will not work in Graph Explorer.
- Can you please ask customer to verify by logging in with the test user (non working) in Graph Explorer to see if they can access the drive items from the test account too?
If the license is not the issue, can you share the inner error details, (request id an timestamp).
If you have any other questions, please let me know.