The modules for the virtualbox guest additions were not rebuilt with the last
kernel update. The previous kernel was 4.1.21 which has working virtualbox
modules.

However, the latest kernel update was 4.1.26, which has no matching compiled
version for the guest additions, so they don't function correctly.

Reinstalling the 4.1.21 kernel reallows the guest additions to work correctly.

So, the bug is more that the rebuild wasn't triggered with the kernel update
rather than that the guest additions don't work.