Thx for your efforts. I tested your scene and the forests do not disappear in there, neither with opening nor closing the render viewport...but they still do in some of my scenes (not in all of them). I tried to reproduce the error myself in new scenes, but didnt manage to do so.
I didn't manage to find the file c:\forestDebug.log. Where exactly should it be stored?
I tested several previous versions of octane and FP in different combinations now to see if the issue is version-related but I didnt figure it out. The forests disappear also with older versions of octane and FP. I never noticed that problem before, that's why I thought it had shown up with the newest versions.
I found out that the forest can be manually updated as soon as the rendering is paused or the render viewport is closed. It disappears again if the rendering is resumed but I can work with that.