I don't know what it means but I also get it each time I open a RAW image & then it goes away when the picture is fully loaded. Nothing seems wrong with the pictures or program?? Just one of those things I guess.
The exclamation mark is a sign that the image is still being processed.
Contrary to jpegs, Raw photos are processed/calculated every time you open them. This takes time. In the mean time you'll see the lowres preview images.