Object Reference issue in changing the Log file name dynamically in threads

Hai
Am creating a web application using Vb.Net for data processing. I am implementing multi threading  in processing functions. And also i used log4net to log the function calls and steps.
My application requires multiple log file. So i used the following code to change the log files dynamically.
''' <summary>
''' Function used to set the name for Log File
''' </summary>
''' <param name="Jobid"></param>
''' <param name="jobtype"></param>
''' <remarks></remarks>
Public Shared Sub SetLogFileName(ByVal Jobid As String, ByVal jobtype As String)
Try
Dim logFileName As String
If (Jobid = 0) Then
logFileName = HttpContext.Current.Server.MapPath("~/Log/Setup_Log.txt")
Else
logFileName = HttpContext.Current.Server.MapPath("~/Log/Job_" & jobtype & "_" & Jobid & ".txt")
End If
ChangeLogFileName("LogFileAppender", logF ...