Power BI Custom authentication: Could Not Load Folder Contents

[claudiocampanile]

I implemented custom authentication in the following way:

1. From the Logon.aspx page I send a SAML Request to an IDP
2. The IDP responds to the Logon.aspx in POST mode with the SAMLResponse parameter
3. I validate the SAML Response, retrieve the user's username from the SAML Response, generate a JWT token and redirect to the same Logon.aspx page with the authToken= parameter in the query string.
4. I return to the Logon.aspx page where: I retrieve the jwt token from the authToken query string, validate the token, retrieve the username and implement the LogonUser of the AuthenticationExtension class to log in to Power BI.
5. Result: the username appears at the top right of the Power BI portal but I don't see any report and the popup in the attached print screen appears.

Making a difference between windows authentication and custom authentication I noticed the following differences:

Windows Authentication
{
"@odata.context": "https://server/Reports/api/v2.0/$metadata#Me",
"Id": "ba230875-437f-40be-9520-f30f2310a123",
"Username": "XXX\YYYYYY",
"DisplayName": "Pippo Pluto",
"HasFavoriteItems": false,
"MyReportsPath": null
}

Custom Authentication
{
"@odata.context": "https://server/Reports/api/v2.0/$metadata#Me",
"Id": "00000000-0000-0000-0000-000000000000",
"Username": "XXX\YYYYYY",
"DisplayName": "XXX\YYYYYY",
"HasFavoriteItems": false,
"MyReportsPath": null
}

finally I noticed that for the call
https://server/Reports/api/v2.0/Folders(Path=%27/%27)/AllowedActions
the Response is empty and I have as StatusCode: 403 Forbidden.

Can someone help me?