I was recently tweakin my Prometheus deployment on Kubernetes when the readiness probe started to fail with a 404.
The issue started when I wanted to add my external URL for Alert Manager. I added the --web.external-url=https://my.url to my yaml:
After a re-deploy, I only had 1/2 containers up (I also run jimmidyson/configmap-reload) and had the following entries in my Kubernetes events
It turns out that when adding --web.external-url=https://my.url you also need to add --web.route-prefix=/
If you're using the helm chart, add this to your values.yaml