Thanks PHYData DBA,
Will look into your suggestion. I've spent a couple of days on this issue, which must be easier than it seems.
What data can I provide to help you help me?
The report data can be found at
John, I think you are getting to what your issues are now.
After reading your last post (made after this one) you are now looking at how your Server is running the report and using the data source.
This has nothing to do with your report or it's code in SDT 2012.
It is about how reports need to access data because of your SSRS settings and configuration.
My recommendations: Create and use Windows Accounts as the Service and Execution accounts. Make sure to read the SSRS set-up guide for how to create these accounts correctly. Make sure to ready the SSRS set-up guide for you version/edition of SQL Server. It's important that your SSRS server is configured correctly. Even the Dropdown control settings are changed by this.