Hi Thorsten,
We tested this tree with VRay distributed rendering, and the GrowFX rendernode works as usual.
I think the problem is that the GrowFX rendernode just not enough time to calculate the mesh for this tree, and as a result a node doesn't start rendering.
Seems this is a peculiarity of the VRay distributed rendering, because the Backburner works correctly, as you noticed...
I can only advise to use the Backburner, or convert your models to VRay Proxies (animated if needed).
Here you have a very hard models, I tried to edit but sometimes got a crashes...
See how we created these hedges: Hedge_1.max
Cheers