Renew Let's Encrypt SSL/TLS Certificate

1) Which certificate was not renewed, the central certificate for mail service and ISPconfig GUI, or a website certificate?
2) If it's the central SSL certificate, have you created a website with the server's hostname, which means a website that uses the same hostname as the system?

 

Ok, in this case, uncheck the Let's Encrypt checkbox, click save, enable it again and click save. If the checkbox does not keeps enabled and you did not get a new certificate after all changes were processed, then follow the Let's Encrypt FAQ:

https://forum.howtoforge.com/threads/lets-encrypt-error-faq.74179/

The most common reason for a website cert renewal failure is that a sub- or aliasdomain of the site is not pointing to the server anymore and this sub - or aliasdomain was not removed from ISPConfig.