Hi Stephen,
I think you are using ASPUpload as the file transfer plugin because this error seems to be related with ASPUpload component and it occurs after applying the Microsoft hot fix Q811114. They have fixed this problem with AspUpload 3.0.0.3, for more information please take a look at:
http://www.aspupload.com/whatsnew.htmlAlternatively, I can provide a workaround (updated AspUpload.asp) for this problem in case you don't have the chance to update the component on your server.
If your problem is not related with ASPUpload, I will investigate your case further.