WIP answer to my own question: .
It seems that Chrome is unstable, caused by the Extension SideWise [which is an otherwise brilliant extension for hibernating windows]… which in turn I think can not handle a crash with any Incognito windows open… or maybe just a crash. Particularly, I was trying to work out what — if anything — is wrong with Chrome, and it crashes when I try to create a new user.
It seems that there is a separate problem with a memory leak — QuickLook (in the Finder) can not handle an EyeTV file [which is actually a package, which is just a folder] where it can not find the thumbnail. (Incidentally, I have tried logging a support call with Geniatech and they are ignoring me.)