we changed the web.confing inserting this line:
<add key="CuteWebUI.AjaxUploader.UseAspNetStream" value="True" />
Our user tested the upload again, on one of the machines affected by the problem. This time he managed to upload a file larger than 150 KB. Then he tried to upload another file, bigger than the previous one (24 MB) and he got a new, different, error:
Does this help you understanding the cause of the issue?
Anyway, this friday I'll pay a visit to our customer and I will perform an upload using the UploaderDebug=1 mode, and report the result back to you. Just one question: in case of error, will I have the time to copy/paste the debug trace, or will the page automatically refresh, removing the contents of the debug panel?