Q:

Invalid printer specified (sometimes it's working, other times it won't)

I'm trying to print from a selected network printer. Sometimes it's working, but other times it won't print, giving me the following error:

"Invalid printer specified.MyCrystalRPTfilename 11124_5324_{67F07633-5EF3-49B4-9573-BB34151D75BA}.rpt" 

I found the different parts of the code below from the net. I know that this was asked here before but the solutions given doesn't work for me, maybe I just missed something.
Try

            Dim PrintDialog1 As New PrintDialog
            PrintDialog1.ShowDialog()
            PrintDocument1.PrinterSettings.PrinterName = PrintDialog1.PrinterSettings.PrinterName

            Dim prtdoc As New PrintDocument
            Dim strDefaultPrinter As String = PrintDialog1.PrinterSettings.PrinterName

            Dim cryRpt As New ReportDocument
            Dim crtableLogoninfos As New TableLogOnInfos
            Dim crtableLogoninfo As New TableLogOnInfo
            Dim crConnectionInfo As New ConnectionInfo
            Dim CrTables As Tables
            Dim CrTable As Table
            cryRpt.Load("C:\path\of\my\report\MyCrystalRPTfilename.rpt")

            With crConnectionInfo
                .ServerName = "myserver"
                .DatabaseName = "mydbase"
                .UserID = "myuser"
                .Password = "mypassword"
            End With

            CrTables = cryRpt.Database.Tables
            For Each CrTable In CrTables
                crtableLogoninfo = CrTable.LogOnInfo
                crtableLogoninfo.ConnectionInfo = crConnectionInfo
                CrTable.ApplyLogOnInfo(crtableLogoninfo)
            Next

            cryRpt.Refresh()
            cryRpt.PrintOptions.PrinterName = strDefaultPrinter
            cryRpt.PrintOptions.PaperSource = CrystalDecisions.[Shared].PaperSource.Auto
            cryRpt.PrintToPrinter(1, False, 1, 1)

        Catch ex As Exception
            MessageBox.Show(ex.InnerException.ToString())
        End Try

A:

Ensure that the printer you intend to use actually exists at the time of the print:
Try checking with this:
if Not PrinterSettings.InstalledPrinters.OfType(Of String)().Any(Function (s) s.Equals(strDefaultPrinter)) Then
  ' Display/handle an error
End If

EDIT
Ok, depending on the version in use, SAP recommends changing to use the PrintOutputController API stating that PrintToPrinter is no longer actively developed or supported:

Reference CrystalDecisions.ReportAppServer.Controllers and CrystalDecisions.ReportAppServer.ClientDoc
Create a Print options object with information about the default printer
Dim options = New PrintReportOptions With
{
    .PrinterName = strDefaultPrinter,
    .Collated = False,
    .NumberOfCopies = 1,
    .JobTitle = report.Name
}
' pass the options to the print method
report.ReportClientDocument.PrintOutputController.PrintReport(options)
' If you're done
report.Close()
report.Dispose()

