So many solutions here
The problem is not about certbot, nor about i-MSCP or its LetsEncrypt plugin but about openSSL v1.0.x which cannot validate the SSL certificates.
To solve the problem, you need in order:
- Make sure that the CA ISRG Root X1 is installed on your system (in /etc/ssl/certs) : PEM AVAILABLE HERE
- Remove the old X3 (mozilla) CA which has expired
kess Good job. I don't think that all steps are required. You don't need to import the full CA. Regarding certbot, I'll update it.
kivamp You're totally right. To solve the problem, install the ISRG root X1 and remove the expired one manually from /etc/ssl/certs
Worked for nearly anything except SMTP, Mail Clients who try to send mails via SSL got Error "Encryptionmethod not supported".