Look into the following links:
- https://www.howtoforge.com/community/threads/letsencrypt-renew.75020/page-2
 - Reconfigure Services by executing a manual update:
 - Tills error FAQ: https://www.howtoforge.com/community/threads/lets-encrypt-error-faq.74179/
 - Debugging stuff: https://www.faqforge.com/linux/debugging-ispconfig-3-server-actions-in-case-of-a-failure/
 - Secure the admin interface with LE: https://www.howtoforge.com/tutorial/securing-ispconfig-3-with-a-free-lets-encrypt-ssl-certificate/
 - With a wordpress site, I had this problem, because of a custom directive: https://www.howtoforge.com/community/threads/lets-encrypt-not-working-for-existing-domains.78868/
 - I had the following error: 
which: no letsencrypt (/root/.local/share/letsencrypt/bin/). I do not know what caused this, but this is the solution which worked for me:ln -s /usr/bin/letsencrypt /root/.local/share/letsencrypt/bin/letsencrypt