Now that you are running from the Report Server, is your connection pointed to the same database? Can you use sql profiler to see the messaging occurring between SSRS and SQL. When the report is run from SSRS what is the parameter value passed to the report server? Then use profiler when running from visual studio, what value is passed from here.

In general, the thought running through my head is the same input in for the same stored procedure should give the same output. So is the input somehow different or is the stored procedure not the same? Is there a permission issue running from report server that is not present when running from visual studio?

Hope this give you some ideas.

Rodney