If the document is read-only (view or comment) mode, or the script is not bound to a document, or the user had installed the add-on but not authorized the permissions, those situations would cause a problem, but other than that, it doesn't seem like you should be getting an error.
And since it worked before, and isn't working now; when that happens I tend to wonder if it's something that Google did.
That issue was just assigned to someone, so maybe it's a real bug that they plan on fixing soon?