> ***************************************************************
> Warning: unsupported pre-release version of X.Org Server installed. Not
> installing the X.Org drivers.
> ...done.
> ***************************************************************
Hi,
Did you upgrade X Server?
It seems that the X.Org Server version 1.*.99 installed on your jessie.
So I think, maybe you should re-execute VBoxLinuxAdditions.run script
after upgrade "xserver-xorg-core" package to latest version.
You can check your X Server's version.
$ Xorg -version
Thanks,
Yusaku