The network rendering nodes don't use any type of license, but the rendering process needs to be started from the workstation with license (to assign job to render slaves using network rendering service - in your case backburner).
So, the render slave can render in "non-interactive" mode as the license is required only to edit or render scenes on interactive Max sessions.
Shortly, with license (workstations) - you can work in "interactive" mode (create, edit, render) x without license (render slaves) - you can work in "non-interactive" mode (rendering assigned by service - backburner, distributed rendering or batch rendering - this service needs to be launched on computer with license).
Please note, that there is also way how to exclude specific workstations for getting a license from the server, changing them temporary to "Stand-alone mode", using "Start Menu->Itoo Software->Forest Pack Pro->Change License Mode". This way is possible to define easily the workstations that will use the available licenses. Hope this make use of network licenses and network rendering more clear.
Anyway, you shouldn't have problems using network rendering. Please double check you are using the same releases on all render nodes. If it's still not working please do a quick test using Forest Tools and do the export of your scattered geometry to instances. You can find it under Utilities panel - More ... - Forest Tools. Please check this reference. This way you can check that the problem happens during Forest object conversion or not.