Vanilla Debian base boxes
These boxes are maintained by Emmanuel Kasper (Debian Developer) and the debian-cloud mailing list folks, but are not an official part of the Debian project.
The default boxes only include software from the Debian main archive and use either rsync or bind mounts for shared folder functionality.
The contrib-* boxes include components outside the debian main archive: namely the vboxfs kernel module for shared folders.
Report a bug
see https://www.debian.org/Bugs/Reporting and report the bug against the virtual package cloud.debian.org