A quick git grep search shows that the XML ID is already in use here:
language/oop5/basic.xml: <example xml:id="language.oop5.basic.class.class.name"> and is the only usage of it (no references) so renaming that example ID could be a solution.

@Crell, the main difference between <example> and <informalexample> is that the former requires a <title> while the latter must not have any (there may also be differences how that is handled by PhD). As such my suggestion was bad. The actual problem have been the duplicated IDs (as pointed out by @Girgias).