I have the same issue with OMP 3.1.2.1 (https://books.fe.up.pt/index.php/feup/catalog/book/978-972-752-260-6). Chapters are correctly presented but without the link to the pdf, which is available (Versão Chapter 1 do ficheiro administrator, 978-972-752-260-6_0001-0004.pdf). In the test environment already tried the upgrade to latest version of OMP but same issue. How is it possible to solve?