The manual in GameStudio has many flaws and LOTS of things aren't described the way they should be, most times i have to google something that the manual should explain
In my experienci, whenever that error ocurred in my scripts i just added a wait(1); to wait 1 frame before the line that was causing this error. So, if it has something to do with the video it may have something to do with DirectX, like Superku said
try googling "lite-c str_width" and search around some websites