Ok, so I have not tried the fix suggested, but here is a bit of an update:
I have just updated all our machines to the latest version of Forest pack and have just had the error occur, so it is not fixed in the latest release of Forest pack. However, for me it is a very specific time, and process that causes this error. In order for me to get this error, all I have to do is: start a local render with v-ray distributed rendering enabled and a selection of nodes being used; then when the process is started, but the nodes have not actually contributed any buckets to the pre-passes, just hit cancel in the v-ray render dialogue to cancel the render. The last time I did this (only a few minutes ago) I got the error locally, although this doesn't usually happen, and on all the nodes. Hitting cancel on this error dialogue will reset the max dummy on the nodes, and you are ready to go again.
In brief, I think it has something to do with the calculation of the forest pack geometry ready for rendering, which is in some way connected to vray, and yet when you cancel the render in the middle of this process it doesn't like it.
I hope that helps.
EDIT: I just realised that this post is in the rail clone forum. I have been having the problem with forest pack, but I assume the error is caused by the same problem. I have both, so don't know which is causing the problem.