Guest desktop services in guests running the X11 window system such as Oracle Solaris and Linux, are provided by a guest service called VBoxClient, which runs under the ID of the user who started the desktop session and is automatically started using the following command lines when your X11 user session is started if you are using a common desktop environment such as Gnome or KDE.

$ VBoxClient --clipboard
$ VBoxClient --display
$ VBoxClient --seamless

If a particular desktop service is not working correctly, it is worth checking whether the process which should provide it is running.

The VBoxClient processes create files in the user's home directory with names of the form .vboxclient-*.pid when they are running in order to prevent a given service from being started twice. It can happen due to misconfiguration that these files are created owned by root and not deleted when the services are stopped, which will prevent them from being started in future sessions. If the services cannot be started, you may want to check whether these files still exist.