Difference about Ajax and REST - ajax

Question

I'm trying to understand a little problem I'm facing in a project that I'm working at the moment.
In my project, i'm the back end developer and I make some functions to be called by the front end with some Ajax functions. Today i'm facing a strange and random problem.
Whenever I use RESTful client to call my endpoints, it works pretty fine, without a single error in any of the individual endpoints. But when the front end calls them using ajax, it sometimes gets an error of "Can't operate on a closed Connection!" or "Can't operate on a closed Statement!", it's pretty random which one of them appears.
I'm using the following structure:
#Repository
public class RandomRepository {
#Autowired
private BasePersistence dao;
EntityManager em;
Session session;
public UserDTO getUserById(Long userId) {
if (userId == null)
return null;
em = BasePersistence.entityManagerFactory.createEntityManager();
em.getTransaction().begin();
session = em.unwrap(Session.class);
User user = session.doReturningWork(new ReturningWork<User>() {
#Override
public User execute(Connection con) throws SQLException {
try (PreparedStatement stmt = con.prepareStatement("SELECT * FROM tbl_users AS u WHERE id = "+ userId +";")) {
ResultSet rs = stmt.executeQuery();
User u = null;
while(rs.next()) {
u = new User(rs.getLong(1),rs.getString(2));
}
return u;
}
}
});
em.getTransaction().commit();
em.close();
return user != null ? new UserDTO(user) : null;
}
}
I use this same structure to make all my database operations.
I create the EntityManager > I begin the transaction > I unwrap the session > I execute a doWork or doReturningWork (depending of the type of query) > I commit > I close the EntityManager.
My question is, even by creating the EntityManager inside the method, could it be closed by another EntityManager created on another method like this by using Ajax to call the endpoints?
If that's the case, how could I prevent this problem?
Is this really a difference between the REST call and the AJAX call?