Renew Letsencrypt Certificate on Nextcloud Box

Hi,

I got a certificate problem on my nextcloud box. “The certificate has expired”.

Effective Date: Tue May 9 12:12:00 2017 GMT
Expiration Date: Mon Aug 7 12:12:00 2017 GMT

Firefox doesn’t allow me to connect to my server because of this, and other apps don’t work as well. The version I have is 11.0.3. It never updated to v. 12, I don’t know if it’s related.
From what I understood, and it seemed to have worked well for the last 9 months, I have nothing to do to renew my certificate.

What can I do ?

Thanks