• I just found an easier way to fix this...

    in my case this has to deal with the following 2 lines in the XML code under the data source connection XML tag:

    <DataSources>

    once you remove these 2 lines

    <rd:SecurityType>Integrated</rd:SecurityType>

    <rd:DataSourceID>95b32f27-f5ce-4e43-87c0-58da9e27a85a</rd:DataSourceID>

    And save the RDL it will open back up in design view...

    Hope this helps

    - Artimus