Visualizer fails to render in its iframe

A workunit with visualizations fails to render any output in its iframe. The resources tab for the WU does show the visualization files, but clicking on /res/index.html results in a blank pane. The browser's console shows:

index.eclwatch.js:2          GET http://university.us-hpccsystems-dev.azure.lnrsg.io:8010/esp/files/WsWorkunits/res/W20220919-063307/res/index.html 404 (Not Found)

This is a cluster without https but with htpasswd authentication enabled.