Installing Guest additions causes graphical glitches in game Return to the Roots

r59145.

Installed Return To The Roots (SFX archive linked):
https://www.dropbox.com/s/abiwgrmebj10bri/settlers25rttr_0.8.exe

Running in VBox without Guest Additions works fine (but is slow), installing Guest Additions changes the status into running fine, but very glitchy (see attached screenshot rttr2_additions.png). Running on 32-bit colors; even if I change ROS colordepth, the game will still look the same.
There's nothing in the ROS log, this is the only potentially useful info in game log:

OpenGL Warning: SHCRGL_GUEST_FN_WRITE_READ (67108924) failed with ffffffdb ffffffea