[DiapDealer]

I'm betting the ones it doesn't work on have a username with a space in it. It doesn't seem to affect other programs configured for "Open With" for other filetypes, so I'm guessing it's something to do with how fontview.exe itself parses a path with a space in it when it's provided as an argument.

I'm not at my computer to test, but it's the only thing that makes sense to me. My work machine's username has a space in it (not my doing). Moving Sigil's scratch directory to a location with no spaces in the path (and/or using an alternate font tool with the default space-infested scratch directory) should quickly prove my theory.