Debian 9 "stretch" fails during startup after today's update, had to disable 3d acceleration

[MeOnVBdotOrg]

I upgraded my 4.9.0-5-amd64 Debian 9 stretch to the latest Virtualbox today on windows 10 host to version Version 5.2.6 r120293 (Qt5.6.2)
Booted the guest no problem, got an alert that the virtual box guest additions are out of date, did the insert guest additions and ran the installer, no issues.
Rebooted and it would start to boot then freeze with a black screen and blinking cursor.
went to console alt-f2 logged in as root, STARTX and got my desktop
went to console alt-f1 logged in as my normal user, issued startx command, got a desktop but it froze after giving a weird error about input output which unfortunately I didn't write down.

Tried a few things without result, could not get a normal boot to come up, came here read through some threads here and seeing something about 3d acceleration so disabled 3d acceleration and now it boots perfectly and starts XFCE.

So something has changed in this update to guest additions that clashes when 3d acceleration is enabled.