We have no similar problems reported. Let me mention, that all Forest objects need to be rebuilt at certain points (load, save, render, etc.). Might be, some of the Forests is taking a huge time to rebuild (perhaps by multiple dependencies of other Forests, collision checking or other causes).
You can check the build of time of each Forest at General->Forest Stats, clicking Display->Build to force a rebuild. I would suggest you try to isolate the problem in a separate scene, removing Forest objects one by one.
Anyway, would you mind to send us your scene to firstname.lastname@example.org to check it here? In case of big file size, please use a web sharing service as WeTransfer. Please specify which 3ds Max / Forest Pack / renderer releases are you using. Thanks in advance for your collaboration, your feedback is very important to us.