This is a problem with your hosting environment, not Akeeba Backup. You will need to ask your host for support.
Your site is using a Let's Encrypt certificate which is signed with an RSA SHA-256 signature. It looks like your host's software is compiled against a version of GnuTLS which does not support this signature method even though it's officially supported per https://www.gnutls.org/manual/html_node/Supported-ciphersuites.html This is something your host needs to address.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!