Anyone running the newest VB 5.2.6 please be aware that installing the built-in Guest Additions on a guest system using 3D acceleration breaks XServer. I.e if you are running Cinnamon Mint as the Guest you won't be able to boot to a GUI - only the TTY.
Once in the TTY it's easy enough to remove the bad Guest Additions module using the command
Oracle have acknowledged the issue - https://www.virtualbox.org/ticket/17463
and have released an updated Guest Additions module (version 5.2.7) for use in 5.2.6 which needs to be manually downloaded from https://www.virtualbox.org/wiki/Downloads