Shared clipboard does not work. VBoxClient --clipboard terminates silently

[Ganton]

When installing Kubuntu 20.04 in a virtual machine, although it has the 6.1.6 version of Guest Additions, shared clipboard does not work.

It happens the same as it's described in [VBoxClient --clipboard terminates silently](https://www.virtualbox.org/ticket/19234):

    $ VBoxClient -v --clipboard -f # Or `VBoxClient --clipboard -d`
    Shared Clipboard: Starting X11 event thread
    Shared Clipboard: Stopping X11 event thread ...
    Shared Clipboard: X11 event thread terminated successfully
    Error connecting to host service, rc=VERR_NOT_IMPLEMENTED
    Service terminated abnormally with VERR_NOT_IMPLEMENTED
    Running service failed: VERR_NOT_IMPLEMENTED

    Previously reported at <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=946843>
    and <https://forums.virtualbox.org/viewtopic.php?f=3&t=96309>

Although that bug was reported for the 6.1.2 version of Guest Additions, the 6.1.6 version in the virtual machine does not solve the bug. The effects are the same using:
- the `dkms`, `virtualbox-guest-x11` and `virtualbox-guest-dkms` packages (and rebooting),
- or `sudo ./VBoxLinuxAdditions.run` from the https://download.virtualbox.org/virtualbox/6.1.6/VBoxGuestAdditions_6.1.6.iso file (and rebooting),
- or `sudo ./VBoxLinuxAdditions.run` from the https://download.virtualbox.org/virtualbox/6.1.2/VBoxGuestAdditions_6.1.2.iso file (and rebooting).

Thank you!

[Ganton (ganton)]

Another user [wrote that "copy-paste between host and client (checked that enabled in config) doesn't work either."](https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1873922/comments/3) "using VirtualBox Guest Utils 6.1.6 and Ubuntu 20.04" (https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1873922).

[Ganton (ganton)]

Another user [wrote that "shared clip board still not working"](https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1871658/comments/5) using the 6.1.6 version of the Guest Additions.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Vlad Orlov (monsta)]

I confirm this.

I've fixed it for myself by installing guest additions 6.0.14 from http://snapshot.debian.org/package/virtualbox/6.0.14-dfsg-4/ (that package has a patch for kernel 5.4 support, unlike the one from Ubuntu 19.10 repos).

[Vlad Orlov (monsta)]

Ok, so this is fixed by guest additions 6.1.10-dfsg-1~ubuntu1.20.04.1 from focal-updates repo (see bug 1882106 for some details of that update).