I'm of the thought that a Windows patch must have broken the permissions and are blocking the script that writes the svg from creating it (but why it works for the fpz is a mystery then), as I can't see something this basic not working at some point in the past. A look at the debug console isn't illuminating, it documents the creation of the fpz file in temp but says nothing one way or the other about the svg (it in fact says nothing after the creation of the fpz which does exist in temp). I may try Fritzing on Linux and see if that works and this is only a Windows issue.
Peter