Using singleton pattern with Entity Framework context - The underlying provider failed on open

I'm trying to add a singleton pattern to my DbContext with Entity Framework. I've always used the singleton pattern for this, and never experienced this error before. I know that singleton is best practice, but if any of you have the time to spare, could you please explain why singleton is best practice?

Problem

Other than that, I get this error:

The underlying provider failed on open

Let's have a look on my code

DAO.cs

public class DAO
{
    private static HourRegistrationEntities hourRegInstance;

    public static HourRegistrationEntities HourRegInstance { get { return hourRegInstance = hourRegInstance ?? new HourRegistrationEntities(); } }
}

Service.cs (example method)

/// <summary>
/// Return a list of all denied Hour Registration for the Login with the given stringId
/// </summary>
/// <param name="stringId"></param>
/// <returns>A list of HourRegistrationDTO</returns>
public List<HourRegistrationDTO> GetAllDeniedHoursForLogin(string stringId)
{
    var id = Int32.Parse(stringId);
    using (var db = new HourRegistrationEntities())
    {
        var dbHours = db.HourRegistration.Where(x => x.LoginProject.Login.ID == id && x.Denied == true).ToList();
        var returnList = new List<HourRegistrationDTO>();
        foreach (var item in dbHours)
        {
            returnList.Add(new HourRegistrationDTO()
            {
                Id = item.ID,
                Hours = item.Hours,
                Date = item.Date.ToShortDateString(),
                Comment = item.Comment,
                CommentDeny = item.CommentDeny,
                LoginProject = new LoginProjectDTO()
                {
                    Project = new ProjectDTO()
                    {
                        Title = item.LoginProject.Project.Title
                    }
                }
            });
        }
         return returnList;
    }            
}

As mentioned, I've ALWAYS used the singleton pattern but NEVER had this error before. What's causing this, as why?