The short answer is to delete a certain certificate from the docker container, per this post here: Http send report errors / duplicati-monitoring - #16 by hairlesshobo
Basically delete this file:
/usr/share/ca-certificates/mozilla/DST_Root_CA_X3.crt
And the run:
update-ca-certificates
Presumably the next release of the official Duplicati docker image will not require this extra step.