How to reproduce:
1. Start a new game with resolution 800x600
2. Import a background 800x500 (not 600)
The relative room size won't be 400x300, but 400x250.
3. Test game
4. Move mouse cursor into the lower black area.
Illegal exception
An exception 0xC0000005 occured in ACWIN.EXE at EIP=0x00414ECA; program pointer is +31, ACI version 2.60.693, gtags (0,36)
The same exception occures when the character walks down into this black area, a GUI should be displayed in this area.
Next test with background 700x500:
Moving the mouse cursor over the right black area, the background from the left side is painted, same for a character walking there.
Thanks for the bug report, I'll look into it.