ForestQt.dll handles multiple functions (Qt Dialogs, Forest Lister, etc.). ForestQtCatalog.dll only manages the Library Browser.
The problem with these DLLs, is that in older releases were installed in the Max root folder. Now, everything is installed at C:\ProgramData\Autodesk\ApplicationPlugins.
When a new Forest is installed using the setup program, it calls to the uninstaller of the previous version. This uninstaller is responsible to delete old DLLs from the Max folder.
But if these files are copied manually (some customers have custom installations), they may be not deleted by the uninstaller, causing the conflict.
The solution is to force a DLL removal of old files in the installer, not in the uninstaller. This is in our to-do list for next updates.