Trouble adding Sugarcube 2.18 to Twine 2.1.1

I'm upgrading to Twine 2.1.1 and Sugarcube 2.18 on Windows. I uninstalled Twine and installed the new version ok, and I've followed the instructions about adding the Sugarcube format.js to Twine, but it's stuck showing "Loading" with the animation. Windows Task Manager shows activity in Twine (>0% and <1% CPU, and about 400kb/s I/O), but it's not moving from that point - I've been waiting over an hour so far.

Have you seen this issue before? I couldn't find anything on the web about it.