VBscript error 800A0005

Question

Although I found similar problems none could provide a sufficient answer to my problem.

I came across an older VBScript that is used for communication and synchronization with Outlook. Somebody (not me) gets a strange error when running the script:

Script:
"Some path"
Line: 286
Character: 9
Error: Invalid process or invalid argument
Code: 800A0005
Source: Runtime Error in VBScript 

Now what I want to know is WHY that error occurs. Just a general interpretation of the error. But first here is the relevant code:

Set refnum = open_outputfile(outputfileverz & outputfilename_short)
  If (errcode = 0) Then
      If (check_textfile(refnum)) Then
          refnum.WriteLine (strOutput)
          errcode = close_outputfile(outputfileverz & outputfilename_short, outputfilename_short, refnum)
          If (errcode = 0) Then
              If (logging) Then 'mit Protokollierung
                  If (check_textfile(logrefnum)) Then
                      logrefnum.WriteLine ("--- " & Now() & " ---" & crlf & outputfilename_short & ":" & crlf & strOutput)
                      If (gBAbort) Then
                          logrefnum.WriteLine (errtext(33)) ' Abbruch durch den Benutzer
                      End if
                  End If
              End If
          Else
                  ok = false
          End If
      Else
          'errdetails = outputfileverz & outputfilename_short wurde schon in open_outputfile erstellt
          ok = False
      End If
  End If 

Line 286 is:

refnum.WriteLine (strOutput)

There are of course two other functions involved (open_outputfile/check_textfile) but they only return a document reference and check if it is a valid reference respectively.

Now I tried to reproduce the error and there is a problem: No matter what I do I don't get this error. I've tried empty references, empty strings, wrote all kind of variable types but I always get other errors.

Can anybody tell me how or why this happens?