Hello,
long story short: update your Joomla installation.
More detailed reply: in 2021 Let's Encrypt organization changed their root certificate. This means that several existing certificates are no longer updated, unless you update your file containing all root certificates. This is something out of our control, since certificates are built in a way to create a chain of trust: if the root is not trusted (not inside the certificate file you have on your system), target SSL connection can not be established, since the certificate seems invalid.
Akeeba Backup uses Joomla built-in certificate, since your installation is rather old, I'm pretty sure that the new Let's Encrypt root certificate is not inside that file. The best solution is to simply update your website and the issue will be automatically fixed.
Let's Encrypt is the largest organization providing free SSL certificates to the public, so you really fix this issue, because it will affect not only Akeeba Backup, but any call that your Joomla website will do.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
