Hi, this error is triggered when Forest cannot load the key "MainDir" from the registry at "HKLM\SOFTWARE\\Itoo Software\\Forest Pack Pro", or the value is empty.
The "MainDir" key defines the Forest installation path (usually at C:\Program Files\Itoo Software\Forest Pack Pro).
Please, use the registry editor in the render node, and check if the registry exists.
If not, for some reason the software is not being installed correctly in the render nodes.