All of a sudden having issues with deploying to GCP using gcloud app deploy
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Created a brand new project and tried to upload a sample hello-world app from G
There was an issue affecting the Flex deployments. The issue was fixed on 07/11 at aproximately 00:40 UTC. As @isharko pointed out, Flex deployments should be working normally now.
There's also a public issue on GCP's issue tracker about this. If new relevant information surfaces, it'll likely be posted there.
Appears it was an issue on GCP's side. The error is no longer occuring, so I assume it was resolved.