We receive certificate expiration notice while we added “Let’s Encrypt” on our websites few months ago (2/3 months).
It’s very difficult for us to spend time on clicking for each website (into our Plesk panel) to renew that certificates.
Questions :
1/ What is the classic expiration delay ?
2/ What occur if we aren’t able to click on “renew” ?
3/ How to automatically renew the certificates ?
4/ Why do we receive notifications for some websites and not for other, while setup dates are the same ?
For Let's Encrypt, every certificate expires after 90 days.
If the certificate isn't renewed, visitors to the site will receive a browser error that blocks them from viewing the site's content.
There should be a way for Plesk to do this for you, and then you shouldn't have any problems with this! However, I'm not very familiar with Plesk and it isn't really a focus of this forum (at least, none of the Plesk developers participate in this forum). You might want to ask about this on Plesk's forum because people there should be familiar with it.
Perhaps there's an automatic renewal feature in Plesk that you somehow activated for one and not for the other?
Also, a common source of confusion about the renewal reminders comes if you originally got a certificate for one set of names, and then later got one for a slightly different set of names. For example, if you got a certificate for example.com, and then later got a certificate covering both example.com and www.example.com, you would still get a renewal reminder for the example.com certificate, even though in your situation it is actually no longer used and no longer necessary. So, you might want to double check whether you somehow have newer certificates with different domain name coverage than the old certificates for which you receive the renewal reminders.