Yesterday I received an e-mail that I had to renew the certificate, so I did by following this guide (https://www.digitalocean.com/community/tutorials/how-to-secure-nginx-with-let-s-encrypt-on-ubuntu-14-04).
I used /opt/letsencrypt/letsencrypt-auto renew to renew and it gave me a message that the renewal was succesful. When I tried to run this command again it even gave me the message that there was no renewal necessary.
When I try to acces my website now, I get the warning from chrome/Firefox that visiting it is unstable. So the certificate still expired…