Let's Encrypt: where are the logs / cronjobs?

Let’s Encrypt module has been working fine for me for a few weeks.

I could install new certificates and it looks like the automatic renewal worked correctly.

Since a few days however, I am unable to install new certificates.
Looking at the expiration date of installed certificates, it looks like the renewal also failed.

So I started to look for logs and for the cronjob(s) responsible of the renewal but I cannot find these.

Where are the activity logs for Let’s Encrypt? So that I can see when it tried to do a renewal, when and why it failed?

Where are the cronjob(s) for Let’s Encrypt automatic renewal? So that I can see when it is going to try again?

I searched a lot for those without success.
The little I could read and understand in /usr/share/webmin/webmin/acme_tiny.py was not enough to find the logs / cronjobs on my system.

Any help welcome!

I think that you can find error on your apache/nginx error logs.