When Firefox (Version 47.0) starts, we get the warning " Configuratie voor de Belgische elektronische identiteitskaart is mislukt. Is de EID-Middleware geïnstalleerd?". The fact is that everything
works normally! One can use the EID-card without any problems. (I managed to fill in my taxes :-) ). Almost 4 users on the linux-mint-forum confirmed this issue.
There is also another cosmetic problem. The link that suggests to install the middleware is broken:
http://eid.belgium.be/nl/Hoe_installeer_je_de_eIDI switched off the warning. So it doesn't bother me any longer, but it might be nice to fix this warning.
If I can do some trouble-shooting, please let me know.