I've been pulling my hair out with this problem.  I can create reports in BI Studio and see them return data. After I deploy them to the Report Server I get this error message when I try to execute the report:

Logon failure: unknown user name or bad password. (Exception from HRESULT: 0x8007052E)

I played with the data source and tried every conceivable credential option.  Here's the fix (which I picked up on another forum):

• Goto Reporting Services Configuration (in Start Menu->Programs->Sql Server->Config Tools.
• Click on Execution Account
• Uncheck "Specify an execution account"
  

That's it. 

I had the same issue on a dev machine - turned out I head used my own domain account as the Execution Account and when I changed my password it caused this error - and the moral of the story is never use your user account when installing software!

cheers

Jeff,

Thanks for this - I had just about run out of hair!