I have converted tree VRayProxy from 30 voxels to 2 as it was suggested and scene rendered fine. I have converted all VRayProxies with big number of voxels and I will test render scene with different plants. I am curious why Multiscatter plugin works fine in this case.
Sincerely, i don't know. Theorically this issue should affect to all plugins rendering proxies as instances, including the ChaosGroup tools (VRayInstancer, ChaosScater).
This solution was proposed by ChaosGroup, when we reported the problem to them. Also it has sense, because in recent V-Ray updates, proxies are generated by default "Optimized for instances" (using one single voxel), as shown in the attached image.
If Multiscatter uses a different technique for rendering proxies, which is not affected by this issue, we don't know.