Once again a link broke. This time when I copy/pasted (CTRL+D) a reflective shader (with unique name). The link that broke was between Lambert and an XYZ shader. Which had nothing much to do with the broken link, I'd say. As it happens I had an XYZ shader outside this container too, so that might have been the trouble. But I can't remember if it had the same name (deleted in the meantime).