An administrator is creating new severs in a public cloud. The workflow creates and application group with three VMs, one web server, one application server and one database server. The three VMs are

If VLANs were misconfigured to prevent inter-tier communication, SSH access to affected VMs would also likely be compromised, contradicting that all VMs are SSH accessible.

A misconfigured proxy typically affects client-to-web server or outbound internet traffic, not internal application-to-database communication which is the source of the 503 database error.

A misconfigured firewall rule is the most likely cause. While SSH (port 22) might be permitted for administrative access to all VMs, firewall rules often block specific application ports like those for database communication (e.g., 3306, 1433) between the application and database servers, leading to connection failures and database-related errors.

If routing were fundamentally misconfigured between the VMs, SSH access to some or all VMs would likely be impacted, or connectivity between specific tiers would fail more broadly, not just for the database port.