Why does this command sometimes throw array bounds exceptions when it contains no arrays - vb.net

Question

I have a process that loops through an array which is a list of PDF filenames, for each file it must write a template PDF which is stored in the same folder as the exe and copy this to a network share as "pdffilename"~001.pdf and then copy the data file from an S3 bucket to the network share as "pdffilename"~002.pdf. In some cases, the file won't exist but we would still need the separator and sometimes the code would just be writing multiple separators (~001) and no data files but when this happens I randomly get failures in writing the separator pdf.
This works fine when running on my laptop through VS but when I deploy the app up to our application server it fails when there are lots of missing files.
If sFile(0) <> "999-Envelopes" Then
Try
boxTriggerReqd = True
My.Computer.FileSystem.CopyFile("doc_sep.pdf", OutputFolder & "\" & Replace(oFileName, "~002.pdf", "~001.pdf"), True)
Log.writeline("Document Separator written:" & vbTab & OutputFolder & "\" & Replace(oFileName, "~002.pdf", "~001.pdf"))
Catch ex As Exception
Call EndTask(ex.Message, "Unable to transfer Document separator", 11)
End Try
Else
envTriggerReqd = True
End If
Try
dlResponse = s3client.GetObject(GetFile)
With dlResponse
.WriteResponseStreamToFile(OutputFolder & "\" & oFileName)
.Dispose()
End With
Log.WriteLine("Data file written:" & vbTab & vbTab & OutputFolder & "\" & oFileName)
Catch ex As Exception
results.WriteLine(sFile(1))
Call EndTask(ex.Message, targetFile, 12)
results.Flush()
End Try
The error log shows
****** S3 PULL ******
Index was outside the bounds of the array.
Unable to transfer Document separator
****** S3 PULL ******
Where it has called EndTask contains the "Unable to transfer Document separator" so it has happened in one of three commands
I'm assuming its the CopyFile command but there is no reference to an array in any of those three. Is it possibly a memory error on the server itself?