At the moment I can't save a pbix on the server, if the server is configured with a custom authentication (form). Even if it would be possible the live connection to ssas would not work because there...
'@Dominik, one thing to make sure is that your custom authentication extension implements the IAuthenticationExtension2 interface: https://msdn.microsoft.com/en-us/library/microsoft.reportingservices.interfaces.iauthenticationextension2.aspx.
