Hi Klaus,
We don't officially service specific, infra-level integrations (like k8s setup/networking), but we try to answer as best as we can.
Assuming you have the whitelists set up properly... I wonder if you are calling the BigQuery endpoint by IP address or by DNS (it's common that DNS record lookups are blocked inside the container).
Apart from this, you may need to go back to basics and see if you can make a vanilla (non-GrowthBook) k8s instance "talk" to BigQuery.