Hi,
though I am doing all UCS-tests on Virtualbox I never tried to use the X-Windows on the system directly on the server during the last years.
Your problem might be related to the missing GuestAdditions. On other systems one would probably try to “insert Guest additions” and run the installer. This needs some preparation on UCS, see UCS 4.1 als VirtualBox Guest: VBoxLinuxAdditions hinzufügen (german) and the generic advisory from https://virtualboxes.org/doc/installing-guest-additions-on-debian/.
Best Regards,
Dirk