A company is migrating its on-premises web application to AWS. The application consists of a stateless web tier and a stateful database tier running on a single server. The web tier uses session data stored in the local file system. The database tier runs on MySQL. The company wants to minimize changes to the application code during migration. They plan to use Amazon EC2 for the web server and Amazon RDS for MySQL for the database. After migrating the web server to an EC2 instance and the database to RDS, users report that they are being logged out of the application frequently. What is the most likely cause of this issue?