upstream uses d3 v5 and d3.arc appeared in d3 v4. So the webui seems to require libjs-d3 >=4 or perhaps libjs-d3 >=5. Then there's #839961 (only d3 v3 is in Debian).
downloading d3 latest and: $ sudo cp d3.min.js /usr/share/rspamd/www/js/lib/d3.min.js fixes the problem. Also the current packaging does not use symlinks for the d3 dependency, because the webui does not seem to support symlinks. Instead, it copies the js files at install time. This means that updates of js deps would not be taken into account when updated. I suggest using upstream files instead which is not ideal but means used versions are reproductible and not function of when the package was installed. Alex
