Cannot startx on FreeBSD10 Current

I'm running FreeBSD 10.0-CURRENT (r251572) amd64 in a VirtualBox VM.

Everything seems to work properly, except that I cannot 'startx'. All I 
get is a blank screen and an apparently hung box. The only way out of it 
is Machine -> Close -> Send the shutdown signal.

The following appears in /var/log/Xorg.0.log:

----------------------------------------------------------------------
<everything looks normal up to this point>
(II) VBoxVideo(0): Using default gamma of (1.0, 1.0, 1.0) unless 
otherwise stated.
(II) VBoxVideo(0): RandR 1.2 enabled, ignore the following RandR disabled 
message.

Fatal server error:
AddScreen/ScreenInit failed for driver 0


Please consult the The X.Org Foundation support 
	 at http://wiki.x.org
 for help. 
Please also check the log file at "/var/log/Xorg.0.log" for additional 
information.

Segmentation fault at address 0x290
----------------------------------------------------------------------

I don't know how to investigate this further. Any ideas?