Connecting external cubes from AX 2012 R3

We are working with AX 2012 R3. We want to connect our SSRS report to external cubes. i.e., cubes not part of AX & not created from AX. We are able to connect to the datasource & create the report. But the following error occurs when opening the report. “Value of the ‘EffectiveUserName’ XML for Analysis property is not valid.” Can somebody assist on how to resolve the error?

make sure your aos service and bc user have rights on SSAS (server administrator rights) and on the datasource which you are using for cube.