Somewhere along the way, I must have tried out the Steam install on my windows machine… and when whatever version it was wanted to update, made the mistake of letting it. The result was an Interface that crashed immediately on startup without anything useful in the logs. As far as I could tell, Sandbox was working correctly. Fixing the mess required uninstalling everything and then doing a clean steam install in advanced mode to wipe out everything. Yah, it is my fault for not keeping straight steam vs native installs, but I doubt I’ll be the only one ever to make that mistake, and it seems that Interface should have been able to give me a little more of a hint what was wrong than just closing.