AWS installation stuck in Read only mode

Yes ElastiCache is Multi-AZ. Setup is:

I’ve just tried to use an entirely different Redis cache and again the build failed.

My app.yml specifies the primary endpoint of the cluster.

So far I have no containers running, just postgreSQL and redis cache.

I am going to try and use a snapshot of RDS this morning with a new Redis cache but if that fails I am not sure what else I can do since I have no containers running to access the UI.

Why did Discourse suddenly go into read only mode without manual intervention?