Although when set up correctly within ispconfig everything updates correctly there are many circumstances where the Lets Encrypt updates fail to renew. (Typically they have renewed but nginx hasn’t restarted to apply the new certificates) Usually this is because of something that has been changed or amended while trying to troubleshoot a certificate problem with one or more domains.
A quick way around this is to add the following line to crontab.
Login to your server and run the following sudo command or login and change to root.
Add the following line to restart nginx every Sunday @ 11pm
0 23 * * 0 /etc/init.d/nginx restart
You can of course change the time and frequency to something of your choosing but once a week seems to be more than adequate.