Issue with Discourse RSS Polling

I am trying to use the Discourse RSS Polling plugin on a self-hosted instance, based on the plugin’s configuration instructions. The background job that polls the RSS 2.0 feed (produced by a Webflow site) raises this exception every time it runs:

I configured the plugin (in /admin/plugins/rss_polling) with a valid URL, valid Author, and valid Category. I left the Tags and the Category Filter fields blank.

(I was able to get posts syndicating to Discourse using the Embedding-based approach from /admin/customize/embedding, but I’d much prefer to use the RSS Polling plugin if possible.)