I found the real reason for the crashes of Ubuntu MATE Welcome on both QEMU/KVM (Virt-Manager) and VirtualBox.
The problematic package is the gstreamer1.0-vaapi (see https://packages.ubuntu.com/focal/gstreamer1.0-vaapi).
It is a dependency of ubuntu-restricted-addons (see https://packages.ubuntu.com/focal/ubuntu-restricted-addons).
So ubuntu-mate-welcome and for example Cheese may be fixed by removing this package with
sudo apt-get purge gstreamer1.0-vaapi
sudo apt-get autoremove --purge # to remove libgstreamer-plugins-bad1.0-0 (is not a problem)
So this is not our bug.