This article discusses about the error “The login failed for data source ‘Unknown’ that occurs with scheduled refresh of pbix report that uses Access database as the data source and the solution to get the refresh to work.
Error message: Data source error: The login failed for data source ‘Unknown’
Clicking on in the status, would give you details of the error:
 -1055784932: DataFormat.Error: Unspecified error. ADB.accdb. ;Unspecified error. The exception was raised by the IDbCommand interface.
Though the test connection to data source succeeds, the refresh fails