In my case, my certificate failed because when Let's Encrypt was trying to confirm the TXT records with my domain manager, Digital Ocean, and for some reason, the TXT record content had double-quotes around them. So I had to login to Digital Ocean, find the TXT record, and updated it by removing the double-quotes at start and end.
I found out this by logging into my Cloudron dashboard - which is expired - using a browser that allowed me to do that. Once I logged in to the dashboard, I renewed the certificate manually. While it was failing (due to extra double-quotes), I opened the log and inspected it, and was able to see that the double-quotes were causing the issue.