OK, so it's just me then. I didn't actually stack them.
What do you get when you just load the light frame in DSS (add to list) and then click on it to see the preview? Does that show data for you? This is where I see it completely black, including the flat.

Yes, they are only 30sec frames. I will ramp up exposure time once I get guiding working.

Thanks for testing this for me, guys! ;)