Since it works in windowed mode and the script doesn't make a difference between that and fullscreen, I assume (hope) that we can blame AGS for it 
So my guess is, that the widescreen borders somehow affect the coordinates of the special drawing surface, AGS is providing for custom dialogs.
I'll have to work on the module in the next days anyway, because Monkey_05_06 has sent me some really interesting code, which needs to be included. So if I'll encounter this widescreenborder problem too, I'll let you know.

So my guess is, that the widescreen borders somehow affect the coordinates of the special drawing surface, AGS is providing for custom dialogs.
I'll have to work on the module in the next days anyway, because Monkey_05_06 has sent me some really interesting code, which needs to be included. So if I'll encounter this widescreenborder problem too, I'll let you know.