We're having the exact same problem here, except one twist:
We use identity impersonate on our intranet. I, as the intranet webmaster have admin rights on our server that hosts the intranet. From my pc, I can upload files when Identity impersonate is true, and I think it's because I have an admin account on the server.
All other users of our intranet that use the CE have this error.
Other than that I wanted to say that the editor is fantastic. We're very satisfied customers, but we're also anxiously awaiting this bug fix as this was a key selling point for us.
Thanks, and keep up the great work!
Adam