Since version 4.2, Sweet Home 3D supports non rectangular shapes for doors and windows, and the shape of the hole in a wall is guessed from the contour of the door. If you want your door to work correctly without trying to find out its SVG shape, simply add a small threshold (pas de porte in French) at its bottom to close it.

importing files using names that include spaces or any kind of non-letter-signs fails invariably - I've had this problem for a couple of months now, but haven't had it before. Did I miss something?

It's a known regression that will be fixed in the next version.