Additional information (FWIW):

 

in my test, I am uploading a 1.4M pdf file (other files do the same) and the server is my own machine so I'm expecting the "network time" for the upload to be practically zero.

 

It is not... After clicking upload and selecting a file, an empty progress bar is displayed (and the text labels indicating progress at 0KB) nothing happens on the client side for about 20-30 seconds and then I get the message "Unable to find the uploaded file...".

during the upload, looking at the server side, I can see in windows explorer that a file does get created in the temp directory, named: uploading.681d5c18-7bd3-fc7c-424b-47d4de77180e.681d5c18-7bd3-fc7c-424b-47d4de77180e.resx (so I assume this means there is no permission problem for the ASPNet/Anonymous user to write to the temp directory) but it just remains at 0 bytes for the same 20-30 seconds. Then it disappears (gets deleted)

 

My application event log has no messages or exceptions logged at the same time.