Export Report to .pdf - report giving problems

The application lets the user bring up time sheets by job number in a grid and be able to select and show the report. There is a page scroll function to scrool thru pages and also a print button that lets you open the report in .pdf and print. For most part is it working but once in a while it does not let us scroll nor print. Below is the code for the print, it errors on the "oStream" line... We are using reference to CrystalDecsisions 11.5.3700.0.

I canno figure out what causes this to happen. Than you.

Code (vs2008)

            myCrystalReport1 = Session("myCrystalReport1")

            Dim oStream As New MemoryStream

            oStream = myCrystalReport1.ExportToStream(CrystalDecisions.Shared.ExportFormatType.PortableDocFormat)

            Response.Clear()

            Response.Buffer = True

            Response.ContentType = "application/pdf"

            Response.BinaryWrite(oStream.ToArray())

            Response.End()

Error:

CrystalDecisions.CrystalReports.Engine.InternalException was unhandled by user code

  Message="The Report Application Server failed"

  Source="CrystalDecisions.ReportAppServer.DataSetConversion"

  StackTrace:

       at CrystalDecisions.ReportAppServer.ConvertDotNetToErom.ThrowDotNetException(Exception e)

       at CrystalDecisions.ReportSource.EromReportSourceBase.ExportToStream(ExportRequestContext reqContext)

       at CrystalDecisions.CrystalReports.Engine.FormatEngine.ExportToStream(ExportRequestContext reqContext)

       at CrystalDecisions.CrystalReports.Engine.ReportDocument.ExportToStream(ExportOptions options)

       at CrystalDecisions.CrystalReports.Engine.ReportDocument.ExportToStream(ExportFormatType formatType)

       at _Default.Page_Load(Object sender, EventArgs e) in C:\Users\vgwprja\Documents\Visual Studio 2008\WebSites\ProjectDailyReport\Show_Pdf.aspx.vb:line 23

       at System.Web.UI.Control.OnLoad(EventArgs e)

       at System.Web.UI.Control.LoadRecursive()

       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

  InnerException: System.Runtime.InteropServices.COMException

       ErrorCode=-2147482651

       Message="The Report Application Server failed"

       Source=""

       StackTrace:

            at CrystalDecisions.ReportAppServer.Controllers.ReportSourceClass.Export(ExportOptions pExportOptions, RequestContext pRequestContext)

            at CrystalDecisions.ReportSource.EromReportSourceBase.ExportToStream(ExportRequestContext reqContext)

       InnerException: