It appears ad blockers such as uBlock and AdBlockPlus block all AJAX requests to the pageviews API. Try uBlock in Firefox and perform requests on [[ https://wikimedia.org/api/rest_v1/?doc#!/Pageviews_data/get_metrics_pageviews_per_article_project_access_agent_article_granularity_start_end | RESTBase ]].
Not sure what is causing this, but these ad blockers are very popular and cause tools like [[ https://tools.wmflabs.org/pageviews | Pageviews Analysis ]] to fail.
See comment from @MusikAnimal below and update the on-wiki docs + the hyperswitch documentation so that others who run into this problem don't get stuck.
Consider making a proxy on the front-end from /pv to /pageviews to avoid this issue, but if ad-block software starts blocking /pv just give up the fight :)
## Possible alternative layouts that aren't blocked
### /metrics/page/*
- `/metrics/page/views/`
- possibly, later:
- `/metrics/page/edits/`
- `/metrics/revision/score/`