The problem seems to be caused by mod_gzip. It dechunks PHP responses, which means that it is storing in a buffer the AJAX responses that show upload progress, instead of sending the responses right away to the browser.
The solution is to tell mod_gzip to not compress that page and so not dechunk responses.
Alternatively, if you use Apache 2.x with mod_deflate you will not have that problem.
This article explains these problems:
phpclasses.org/blog/post/58-Respons
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