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…
Did you reload nginx after renewing your certificate? Nginx reads the certificate files at startup and will need to be reloaded when they change (service nginx reload will do).
If you did restart nginx and the certificate still shows up as expired, please share the nginx configuration related to SSL, and the output of ls -l /etc/letsencrypt/live/example.com, where example.com is the directory in which your certificates are stored.