Created Unassigned: calling FirstOrDefault() function hangs forever [14963]

Hi,

What I observed is when first request is still executing the method FirstOrDefault() and second request also tries to execute same method, it hangs foreever :(

__Code:__

using (OfficeOpenXml.ExcelPackage excelFile = new OfficeOpenXml.ExcelPackage(stream))
{
Trace.WriteLineIf(reached here -1");
__ OfficeOpenXml.ExcelWorksheet sheet = excelFile.Workbook.Worksheets.FirstOrDefault();__
Trace.WriteLineIf("reached here -1");

................................
..................................
}

It is an ASP.NET web application and Framework 4.0 and EPPLUS ver 3.1.3.
Anyone ever faced this or have an idea how to get this working ? Thanks.