GitLab + Kubernetes - Error about project billing

I have successfully set up GitLab oauth2 integration with GCP, but when I try to create a new staging k8s cluster I get the following error message:

‘We could not verify that one of your projects on GCP has billing enabled. Please try again.’

I have 2 clusters running in the project already, and therefore have billing associated with that project. I have tried creating a new project, linked to the same billing account, but still no luck.

Any clues as to why this might be happening?

I am experiencing the same error. My Gitlab installation is behind VPN - could it be the reason?
How to debug it?