We've received your scene and did testing with it. Most probably the plug-in is not properly installed on some of your render nodes or it's from a different version. Please double check you are using the same release on all render nodes
(also same 32/64 bits version
If so, I would suggest you for each slave node:
1) Be sure 3ds Max and VRay Spawner are stopped.
2) Delete ForestVRay150sp3.dll
from the Max root folder (some files may not exist, depending of the VRay version), and ForestPackPro.dlo
3) Reinstall Forest Pack using the setup program, or manually as described here
That should fix it.