I’m assuming this is an initial setup. If it’s a sudden breakage, then the below might be off-target.
Http send report errors / duplicati-monitoring and other posts there and in the forum discuss this.
So far that third-party (not by Duplicati team) service has stayed with default Let’s Encrypt chain.
A workaround from you is typically deleting expired certificate in either your OS or the mono level.
DST Root CA X3 should go away. It’s expired, but still helping some systems, but breaking mono.