What is the cause of this error?

Explanations:

If the user name and password were incorrect, the application would typically receive an authentication error rather than a timeout. A timeout indicates that the application cannot reach the database server, suggesting a connectivity issue rather than credential problems.

If the security group assigned to the application servers did not allow outbound connections to the DB instance, the application servers would not be able to initiate connections to the database. However, since the connection is timing out, it indicates the application servers are attempting to connect but cannot reach the DB instance, which points to a possible issue with the DB instance’s inbound rules.

The timeout error suggests that the DB instance is not accepting incoming connections from the application servers, likely due to security group rules on the DB instance that do not permit inbound traffic from the application’s IP addresses. This is a common misconfiguration that would result in connectivity issues.

If the user is not authorized to use the DB instance, the application would receive an authorization error, not a connection timeout. A timeout typically means that the network connectivity to the DB instance is blocked rather than an issue with user permissions.