This is a really strange bug, because I can load the STL file just fine. But somebody else reported a file to me that they couldn't load (it would bail out with the same error), and it seems like this is a known issue.
I just wonder what's different on my machine that it doesn't have this problem? It's a long shot, but do you have python3-stl installed? I have marked it as a Recommended dependency in python3-uranium, so it might not be installed on every system.
