I'm having the same issue. Just upgraded the department, 24 slaves and 4 workstations. Forest works on the slaves if I do not use there preset libraries. When I do use the libraries on the render farm it only works on other workstation, not the slaves.
This is basically a problem of the DR nodes, which for some reason cannot found the references used by Presets (as the XRefs or the materials). Anyway we don't use nothing special here: once applied, a preset is a standard Forest object, with their materials and custom objects.
In case the library is working in XRef mode (there is a selector for that), the Preset object is added to the scene as a XRef. It not, it is merged. And all the presets objects are installed with the setup program at C:\Program Files (x86)\Itoo Software\Forest Pack Pro\lib\Presets, so they should exist in the slave nodes, if Forest have been installed on them.
We are trying to reproduce the problem here, but our tests run fine. We don't know why certain configurations of DR fail to locate the preset references (xrefs, materials or something else). We also are in contact with ChaosGroup about this issue, i hope we have more information soon.
By now, the only workaround is to use the Presets in "Merge" mode, or merging the objects to the scene if you have imported them as XRef. Unfortunately, this seem to not work in all cases.