Looks like CUL has gone through a page redesign. Some of the DOM structure has changed, which has broken my GM script for adding the custom 1-click-add-to-my-library buttons.
The problem has to do with:
// old format:
//var links = document.evaluate("//li/a[@class='title']",
// new format: they added a layer of DIV
var links = document.evaluate("//div[@class='list']/li/div[1]/a[@class='title']",
Now it works again.