I too was having the same issue, albeit on an Arch Linux system. What finally allowed to fix it are the explanations given here: SSL certificate errors · KSP-CKAN/CKAN Wiki · GitHub
More precisely, banning the DST X3 Root Ca and then forcing to update system store and mono store did the trick for me.
Here are the commands I used:
sudo trust extract --format=pem-bundle --filter='pkcs11:id=%C4%A7%B1%A4%7B%2C%71%FA%DB%E1%4B%90%75%FF%C4%15%60%85%89%10;type=cert' /etc/ca-certificates/trust-source/blocklist/untrusted_authority.pem
sudo update-ca-trust
sudo cert-sync /etc/ssl/certs/ca-certificates.crt
The first one bans the expired root CA and the two others update the required certificate stores. In particular, the last one showed this in its output:
Certificate removed: O=Digital Signature Trust Co., CN=DST Root CA X3
Once I got this, my backup started working again with a Backblaze B2 remote.