Hi there
These errors you're experiencing are no doubt caused by the security 'hardening' introduced to VirtualBox.
This is causing a lot of concern to users of the software whose VM's are now 'broken'.
You can follow the progress towards a solution at the VirtualBox forums, specifically this one ..
https://forums.virtualbox.org/viewtopic.php?f=6&t=66639
In the meantime to get your VM's working again, you will probably need to revert back to version 4.3.12 (which was the last one before 'hardening' was introduced).
This has been an ongoing problem for a long time and I personally can't wait any longer for VirtualBox to be fixed. I intend to switch to VMWare in a few days.