I have installed a vanilla ROS 0.4.12 RC17 on VBOX 6.0.6 with the settings described on this wiki page. Reboot works reliably.
Next thing I typically do (and it worked in previous releases of ROS) is to install the AC97 sound driver, assign it to the devide and do a few reboots.
On my very first attempt to install the driver the reboots worked but sound did not, just as noted in CORE-15969. After restoring a snapshot and doing the AC97 installation from there again, BSOD appeareed during almost all reboots.
One of the messages shown in screen debug mode are:
thread.c:119) PS: Unhandled kernel mode exception
Note this is the first time I use VirtualBox 6.x. An older shaphot with AC97 just installed in ROS 0.4.10 RC22 prior the update to VBox 6 now also produces this BSOD on VBox 6 with equal debug messages - and this was working on VBox 5.1.22.