We can suggest the following actions to narrow down the problem:
- Replace the upload with a regular input type="file" in a form. Post the form using regular submit button
If you get the same behavior then we can be sure that the problem is in the Upload widget itself.
- Try a direct upload If using a proxy
- Use Fiddler and/or Wireshark to observe and record the failed network sessions.
I hope this will give us some clues on where to look next.
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