I was having issues with this as well… not just with the http send report, but also with performing the backup itself because my backup destination is a WebDAV server that uses StartSSL.
On Debian 10, I was able to work around this problem by performing the following:
##!!! use at your own risk.. I do NOT suggest deleting the offending cert
##!!! but instead moving it to another location in case you need to put
##!!! things back the way they were for some reason.
# update apt
apt-get update
# make sure the latest ca certs are installed
apt-get install ca-certificates
# remove the offending root cert from the root store
mv /usr/share/ca-certificates/mozilla/DST_Root_CA_X3.crt ~/DST_Root_CA_X3.crt
# update the root ca store.. this automatically updated mono for me.
update-ca-certificates
After I performed the above steps (as root or with sudo), my backups started working again. Didn’t even need to restart duplicati.
This may come around again in the future, but for now at least I can backup my systems.
Hope this helps someone!