In V-Ray 6 the SDK is new, so all 3rd party plugins may not work and even crash (ForestPack, RailClone, Multiscatter, etc) If there are problems, you might need to uninstall those for the respective 3ds Max version.
If you want to use such plugins with V-Ray 6, you'll need an updated compatible versions from the respective software developers.
If you want to use such plugins with V-Ray 6, you'll need an updated compatible versions from the respective software developers.