What was going on
Upgrading Flipt from 1.8.3 to 1.9.0 or 1.10.0 breaks Postgres connectivity in Kubernetes with: getting db driver for: postgres: dial tcp: lookup postgres.database.svc.cluster.local: device or resource busy. Versions 1.8.2 and 1.8.3 worked fine, and the error appears even against a fresh database, so it is a regression in the 1.9.x connection handling.
What fixed it
This was a Flipt regression fixed in v1.9.1 and v1.10.1, upgrade to either of those patch releases and the Postgres connection works again. The reporter confirmed v1.9.1 fixed it in both local Rancher Desktop and GCP clusters. If you must stay on 1.9.0/1.10.0 temporarily, rolling back to 1.8.3 avoids the error.