Although I've had this problem since at least Pacific, I'm still seeing it on Reef. After much pain and suffering (covered elsewhere), I got my Prometheus services deployed as intended, Ceph health OK, green across the board. However, over the weekend, the dreaded "CephMgrPrometheusModuleInactive" alert has returned to the Dashboard. "The mgr/prometheus module at dell02.mousetech.com:9283 is unreachable." It's a blatant lie. I still get "Ceph HEALTH_OK". All monitor status command show everything running. Checking ports on the host says it's listening. More to the point, I can send my desktop browser to http://dell02.mousetech.com:9283 and get a page that will allow me to see the metrics. So everyone can see it but the Dashboard! I did have some issues when the other prometheus host couldn't resolve the hostname, but I fixed that for all ceph hosts and it was green for days. Now the error is back. Restarting Prometheus didn't help. How is the Dashboard hallucinating this??? Tim