I've figured out the problem here - env/lib/python2.7/site-packages/xstatic/pkg/svgedit_moin/data/editor/extensions/ext-server_moinsave.js has a line:
var name = qstr.substr(9).split('/+get/');
Which should be:
var name = qstr.substr(9).split('/%2Bget/');
As Flask's url_for quotes the URL so it can be included in the form.
Presumably it was broken by an upstream change in Flask or Werkzeug.
Also: the "Unable to load from URL:\nundefined" error message is unrelated, undefined refers to the fact that jQuery did not return an error message when it got a 404 on the /+get/<item name> page. See line 4813 of svg-editor.js.