jQuery/PHP upload above certain size fails, but only for certain users

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I use a jQuery upload script with PHP. It works fine. Users can upload files of up to 2GB. I have one user struggling to upload files. I tested it remotely on his computer, and anything over 500KB does not even make it to my PHP script. The same file uploads fine from my computer, using the same browser. I'm suspecting a setting on the user's side, maybe in the browser (chrome), but I don't even know where to begin. I'm not posting any code, since the code is fine for most users - and it's a bit involved to post here. Thank you.