Old Let’s Encrypt Root Certificate Expiration and OpenSSL 1.0.2 is probably the problem.
Although your OS is maybe on 1.1.1, mono’s SSL is BoringSSL forked at OpenSSL 1.0.2.
Some vendors have (over time) removed the offending certificate. Maybe yours has not…
Please make sure the OS certificates are up to date, but you might need manual removal.
The error will find scattered OS-dependent methods, and I’m not sure what fits yours best.
The ideal solution would be resistant to a future OS update (not) helpfully undoing remove.
Http send report errors / duplicati-monitoring is midway in one discussion that might assist.