Thank you for your response. As usually happens I got side tracked and now I am back to this issue.
It still occurs. We did try a newer version. I still have to try the change to the web.config as you mentioned.
BUT I fired up Chorme developer tools and looked at the console when uploading that first file. I get the same results as the mobile clients do, the first file fails. But iknow I see an error message!!! Maybe you can tell me what it means!