At least one of alertmanager cluster members cannot be found.
Check if IP addresses discovered by alertmanager cluster are the same ones as in alertmanager Service. Following example show possible inconsistency in Endpoint IP addresses:
$ kubectl describe svc alertmanager-main Name: alertmanager-main Namespace: monitoring ... Endpoints: 10.128.2.3:9095,10.129.2.5:9095,10.131.0.44:9095 $ kubectl get pod -o wide | grep alertmanager-main alertmanager-main-0 5/5 Running 0 11d 10.129.2.6 alertmanager-main-1 5/5 Running 0 2d16h 10.131.0.44 alertmanager-main-2 5/5 Running 0 6d 10.128.2.3
Deleting an incorrect Endpoint should trigger its recreation with a correct IP address.