I have an issue that I think is related. My game uses http requests to save/load games to a server of mine, but such requests sent by builds hosted on itch.io always fail.

To make sure this wasn't a problem with my scripts, I made a tiny test app that sends a POST to http://httpbin.org/post. This is what it returns when I run it locally (and I get something similar when it's hosted on my own site, whether or not it's embedded in an iframe):

sending request...
sending to : http://httpbin.org/post
with fields hash=1337&name=test&version=404
Http POST complete!
Status=200
ResponseText={
    "args": {},
    "data": "",
    "files": {},
    "form": {
    "hash": "1337",
    "name": "test",
    "version": "404"
},
"headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip, deflate",
    "Accept-Language": "en-US,en;q=0.5",
    "Connection": "close",
    "Content-Length": "61",
    "Content-Type": "application/x-www-form-urlencoded",
    "Host": "httpbin.org",
    "Origin": "http://localhost:56116",
    "Referer": "http://localhost:56116/MonkeyGame.html",
    "User-Agent": "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:55.0) Gecko/20100101 Firefox/55.0"
},
"json": null,
"origin": "24.21.224.8",
"url": "http://httpbin.org/post"
}

But when I host & run the exact same thing on itch.io, I get:

sending request...
sending to : http://httpbin.org/post
with fields hash=1337&name=test&version=404
Http POST complete!
Status=0
ResponseText=

So maybe y'all aren't changing our uploaded files at all, but are you somehow blocking outgoing links/http requests? A security thing, maybe?