maybe that's coz it's just wayyy too underexposed.
underexposed or decent exposure is not the problem here. at iso6400, i can still see a very dim image on the times it work. on the ones that show Err. the image is black. black as in "lens cap on" black.
LOL...i can understand why you're saying that. though this is my first dslr for a week, I've been using dslrs for a long time.
my problem now is, sometimes the mirror/shutter does not complete the whole cycle. let's not talk about underexposed or not. what i'm getting is black. total black like as if u had the lens cap on.
this Err problem happens intermittently but quite frequently. on the times it snaps correctly, I get an underexposed image or a normal image. on the times it fails (Err), i get a "lens cap on" black image.
now it doesnt just happen at 1/8000, it randomly happens at lower shutter speeds like 1/60, 1/40. ISO is at auto, so on the times it work, i get a normal image, on times it fail, i get black image but exif still shows the actual shutter speed i set.
when top LCD shows Err. i switch it off, took out the lens, saw the mirror was up and the shutter was covering the sensor. put it back, switched it on, press the shutter button again, and i can hear the mirror flip down. now I can continue snapping.
sorry if this is real long but i cant really think of a better way to explain this as I dont have a video of it.

Did you by any chance set the mirror lockup, mirror up for cleaning, or shutter delay function, either accidentally or otherwise? Or maybe assign a custom button function to these settings and accidentally pressed it?
If not... then it would appear to be a mechanical issue. You can try taking pics with the lens off body and have a visual look to see if and when it happens, what actually happens.