How to select a child entity that is referenced by n parent entities in a ManyToOne relationship?

I have two entities:

@Entity
public class Task {
  @Id
  private long id;

  @ManyToOne(fetch = FetchType.EAGER)
  private Employee employee;
  //... omitted for brevity
}

@Entity
public class Employee {
  @Id
  private long id;
  private String name;
  //... omitted for brevity
}

Now I want to select all the employees that are assigned to at least minTasks tasks.
I tried it like this:

CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<Employee> criteria = builder.createQuery(Employee.class);
Root<Task> root = criteria.from(Task.class);

//Join<Task, Employee> join = root.join(Task_.EMPLOYEE);
//criteria.select(join);
//criteria.groupBy(join);
/*criteria.having(
    builder.greaterThanOrEqualTo(
      builder.count(join),
      minTasks
    )
  );*/

criteria.select(root.get(Task_.EMPLOYEE));
criteria.groupBy(root.get(Task_.EMPLOYEE));
criteria.having(
  builder.greaterThanOrEqualTo(
    builder.count(root.get(Task_.EMPLOYEE)),
    minTasks
  )
);

Query<Employee> query = session.createQuery(criteria);
return query.getResultList();

This fails with the message:

Column reference 'Employee_.ID' is invalid, or is part of an invalid expression.  For a SELECT list with a GROUP BY, the columns and expressions being selected may only contain valid grouping expressions and valid aggregate expressions.

But I do not really get what this message is trying to tell me. Can anyone point me in the right direction?

I would refocus the query as follows, since you have to have as many parameters in the group by as there are properties in the Employee entity:

CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<Employee> criteria = builder.createQuery(Employee.class);
Root<Employee> root = criteria.from(Employee.class);

//Subquery to exists
Subquery<Long> numOfTasksSQ= criteria.subquery(Long.class);
Root<Task> rootSQ = numOfTasksSQ.from(Task.class);
Join<Task,Employee> joinSQ = rootSQ.join(Task_.employee,JoinType.INNER);
numOfTasksSQ.select(cb.count(joinSQ.get(Employee_.id)));
numOfTasksSQ.where(cb.equal(joinSQ.get(Employee_.id),root.get(Employee_.id)));

criteria.where(cb.greaterThanOrEqualTo(numOfTasksSQ.getSelection(),MIN_TASK ));

criteria.select(root);

the result would be a query like this:

select e.*
from employee
where (
    (select count(e2.id)
    from task t
    inner join employee e2 on t.employe_id = e2.id
    where e2.id = e.id) > MIN_TASK
);