A company uses a two-tier architecture with application servers in the public subnet and an RDS-based database in a private subnet. The data engineering team is able to use a bastion host in the public subnet to log in to MySQL and run queries from the bastion host. However, end-users are reporting application errors. Upon inspecting application logs, the team noticed several "could not connect to server: connection timed out" error messages. Which of the following would you identify as the root cause behind this issue?