I figured out what triggers the bug: I set up this artwork in Illustrator and then flipped everything horizontally before exporting. Without the flip, no error. So I’m guessing what happens is that it’s saving as a rasterized image with a transform applied to it, which seems to hit a bug in the Glowforge processor. If I re-rasterize the image after flipping it, I am able to successfully process the file.
So yes, “I got it working”, but please ensure there is a bug filed internally for fixing this. It should be easy to make a reproducible test case. If you need help with that, I can do it.