Thanks. I tried to open the project, but I get that white screen again. Here’s a video:
(nope, can’t upload it. I get a message saying “Sorry new users can’t upload videos”).
So I guess you’ll have to take my word for it! 
Should I send it to the support e-mail address?
I thought that maybe the problem was that I’ve been trying to open a file that I had first imported into AutoCAD Mobile and then tried to open in Shapr3D. I tried importing the original file, as I received it from someone who had created it in AutoCAD desktop, but I can’t open that one either. I get the "white screen of death.
When I first imported the file (I could access it then), I used the Extrude command and tapped on the walls, but nothing happened. But if I tapped on an area within the walls, I could extrude it.