Yes that’s correct on a web browser but if you run monos certmgr -ssl -c https://duplicati-monitoring.com it will show DST_Root_CA_X3 in the chain.
I applied the same fix to a Centos 7 System but cert-sync did not totally remove the offending certs.
certmgr report the DST cert not installed but on future investigation physical cert files where in the store…
Had to take a brute force option
Mono certs stores are in .~/.config/.mono/ or /usr/share/.mono depending on setup
The folders may have certs in either or in my case both certs\Trust and new-certs\Trust. I had to grep to find the DST cert files and move/remove them.
You dont have to run a backup to test as checking for an update will throw the same error