I would suggest you convert all Forests to stand-alone objects using Forest Tools, and then repeat the test with Forest installed and uninstalled.
If the Forest objects are too large to be converted, please use a minimal scene where you can reproduce the problem.
To uninstall Forest, it's not necessary to remove all. Just move temporary ForestPackPro.dlo from "plugins" to any other folder.
If it crashes or generates any strange result with Forest disabled, i think it's time to ask to ChaosGroup again.
If you can get any minidump in the process, please upload it and we'll check it.