A company has deployed an e-commerce web application in a new AWS account. An Amazon RDS for MySQL Multi-AZ DB instance is part of this deployment with a databa...

A company has deployed an e-commerce web application in a new AWS account. An Amazon RDS for MySQL Multi-AZ DB instance is part of this deployment with a database-1.xxxxxxxxxxxx.us-east-1.rds.amazonaws.com endpoint listening on port 3306. The company's Database Specialist is able to log in to MySQL and run queries from the bastion host using these details.
When users try to utilize the application hosted in the AWS account, they are presented with a generic error message. The application servers are logging a `could not connect to server: Connection times out` error message to Amazon CloudWatch Logs.
What is the cause of this error?