Might be BoringSSL Bug: Cannot connect to websites using let’s encrypt SSL certificates #21253.
Old Let’s Encrypt Root Certificate Expiration and OpenSSL 1.0.2 adds background to mono issue.
Because mono has no plan to change their handling, changing certificate store is the workaround.
Old Debian is image’s base, and ca-certificates/20211004 (in testing and beyond) would fix this.
Until Duplicati picks that up or fix propagates further into the old Debian releases, maybe use this:
I would guess that you then need to run update-ca-certificates, but you could test that theory.