Oh no, I didn't mean you, gpau. That was a message for Moini. (And Inkscape developers.)
And Moini will have a better chance of understanding the Traceback info than me. If she doesn't answer pretty soon, I'll give it a try.
When you said that signcut should open when you go to Extensions menu > Export > Send to SignCut, does that mean you've used SignCut with Inkscape before? Have you just upgraded something (Inkscape, Mac, the Inkscape extension, or maybe Python)?
My best guess about the Traceback is that something (a file called inkscape.svg) is not where it's supposed to be. And that could mean that something isn't installed correctly. But that is a guess.
But let's see what Moini thinks first, and we'll go from there.