We have the same problem. Vray Next for 3ds Max (2018.4), Forest Pack 6.1.7, "Forest Pack is not compatible with the current VRay version" error when rendering.
After the rendering is finished (with no FP objects), FP follows with "Some Forest objects were not built correctly in the last render. See 'View Stats' to get additional information about the error: <FP object>". Stats says that the maximum number of faces exceeded in the last render, but adjusting that param in the Display tab does nothing to help, the face number is always exceeding the set maximum.
The thing is, FP used to work with the same 3ds Max + VRay setup just a while ago, but the 6.1.7. update seems to have broken that.