Reminder....

After the last Virtualbox update I wanted to update also the guest additions because I never did before.
Due to the backport sources there could be a new kernel.
Furthermore I got some hint about libgl need a fix.

So here is what I did:
as root:

It is also necessary that you check your kernel version (uname -r) and if the headers are missing install the headers for your running kernel.
After this you can install the guest additions.