Have you tried either of these solutions?
First, there is apparently an environment variable that you can add. I have never tried this solution myself.
community.dynamics.com/.../146289
Second, you can use FullTrust in the SSRS configuration as described in the following link. This is also commonly use to solve a number of other issues that can occur at the same time. I've implemented this solution many times.
yetanotherdynamicsaxblog.wordpress.com/.../ssrs
Hope this helps.