This a typical error of incompatibility with the VRay version.
VRay 2.10.01 is a bit old, and probably ChaosGroup introduced some changes in their SDK on recent versions. If i remember well, Forest Pro 3.8.1 was compiled with VRay 2.20.
Please, upgrade to VRay 2.20 or 2.30, that should solve the problem. You can download the update freely from the ChaosGroup site.