It is more than likely the .NET security on the server machine that is causing the problem.

.NET uses code access security which by default does not allow access to other machines unless otherwise granted.

GO to the machine where reporting services is installed, go into Control Panel-Administrative tools. Find the .NET Framework Configuration tool.

Select My Computer... Runtime Security Policy... Then Adjust Zone Security...
Next Make Changes to this computer...Select Local Intranet from the list of zones... Grant that full trust. Finish out the wizard.

See if that does the trick.

Rodman