There are two bugs that can be seen when installing Xubuntu 24 in a VirtualBox VM:

  1. The first login always fails after a reboot
  2. After installing the VBox Guest Additions, you will see an error message saying “VirtualBox Client cannot connect to X server”

After a lot of research, it seems it’s just a bug in Xubuntu. This post (in French), shows a workaround towards the end:

$ sudo apt remove libva-wayland2

This seems very ugly, but at least the two above bugs disappear.