Can't edit datasource in PBIRS with custom authentication

Hi all,

I have set up a server with PBIRS Jan2020 and custom authentication following the github pages indications.

I can connect to the server using the forms authentication and everything seems to work fine.

Main problem comes when i upload a PBI Report to the server with a SSAS live connection, i can't edit the datasource in the management tab of the report, the option datasource is disabled so i can't change the user and password to connect to ssas.

On the other hands, with default windows authentication works without any issue.

I tried uploading the report with the PBI Desktop > Save As > Report Server option and uploading manually the report to the server and in both cases i can't edit the datasource.

In the server logs i found this line: 

Failed to get connection string for model | RequestID = 29a6f4bd-a6ce-28a6-0b07-59a3380b4db6 ClientSessionID = 35485d15-aa30-e08e-e29b-af7bb15732c1 Microsoft.PowerBI.ReportServer.AsServer.AsConnectionException: Failed to get connection string for model

Any clues?

Thanks and regards