You support an application deployed on Compute Engine. The application connects to a Cloud SQL instance to store and retrieve data. After an update to the application, users report errors showing data

Serial port logs are primarily for low-level system and boot diagnostics, not for troubleshooting application-specific database timeout issues after an update.

After an application update, Cloud Profiler can visualize the application's resource utilization (CPU, memory, I/O, contention), helping to identify if the update introduced inefficient code, slow queries, or contention that causes database operations to time out even with a stable user count.

While increased connections can cause timeouts, the question states active users remained stable, making an application change (diagnosable by profiler) a more probable cause for connection-related issues like leaks or inefficient use rather than just an overall load increase; Cloud Monitoring would typically be used to check this metric.

Cloud Security Scanner is for security vulnerabilities, and a DDoS attack is less probable as the primary cause for database timeouts occurring specifically 'after an update to the application' compared to an application-introduced performance bug.