The engine seems to detect the highest possible resolution even if the resolution is slightly larger than the monitor resolution. To fix that issue for all Adventure Games Studio games, open the acsetup.cfg file and change:
gfxfilter=max
to:
gfxfilter=StdScaleX
where:
X is a multipler of a base-resolution 320x240, ie: setting gfxfilter to StdScale2 will set the game resolution to 640x480.