I tracked down the problem. These two packages are missing from the Docker images:
libmono-system-net-http-webrequest4.0-cil
libmono-system-web4.0-cil
Edit:
You can “fix” the images by running these two commands (assuming your container is named duplicati):
> docker exec duplicati apt update
> docker exec duplicati apt install libmono-system-net-http-webrequest4.0-cil libmono-system-web4.0-cil -y
Then restart the container and all should work. I will update the docker scripts and build a new experimental.