After WPML support looked at it in the English version of the site, the following occured:
In English version everything is fine.
Spanish version has two projectpages for "Beachhouse Málaga" (one with WPML in the name) and projectpage "facelift small apartment (Cambio de imagen de un apartamento)" is missing completely on the Spanish homepage.
Order of the projects: Projectpage Beachhouse Málaga is on second position in the Spanish version. In English version it is the first project (which is correct).
In the back-end it is not possible to open the projectpage "Facelift small apartment projectpage" in the advanced translation editor.
There seems to be something wrong with the linking of projects between English and Spanish version after WPML support has looked at it. Please resolve this asap!! This site has to go live today!!
Pagination is not visible on translated projectpages.
Eventually I made the project pages from scratch, first in English, after this translated them in Spanish and deleted the old ones. Because I really needed to continue with the site. There are still some issues, but I have a support ticket for that, hopefully that can be solved soon.