There are multiple versions of VRay 2.0. Please, see the exact number at the start of the vray log, or in the caption of the render settings window.
Anyways, this is clearly an incompatibility with the VRay installed. There are two possible causes:
- By some reason, the file ForestVRay20.dll was not updated correctly by the installer. Please, remove this file from the Max root folder, reinstall Forest and try again.
- Forest 3.8 is compiled using the SDK of the latest VRay release. It is possible that ChaosGroup changed something between versions, introducing some incompatibility with the compiled plugins (this happened other times). In this case, please, upgrade VRay to the latest version (now 2.20.03) . You can download it from the ChaosGroup website. That should solve the problem.