Switched from latest version of Chrome to latest version of Firefox … opened the SVG in Inkscape and then renamed and resaved … viola --> it’s alive and working.
I find this happens to me when I access the web site from multiple browsers. It can usually be ignored (although it might drop you back at the home page). You might even find your file uploaded successfully.
It’s been a little while since I’ve seen any replies on this thread so I’m going to close it. If you still need help with this please either start a new thread or email support@glowforge.com.