Page Not Found when executing a popup screen?

I have an input element that's configured as a button (see code below). When clicked, it opens a popup window.

It works perfectly on almost every site its installed on. However, on this particular site, whenever its clicked, it goes to the WordPress "page not found",开发者_如何学Python 404 page.

What could cause this?

<input 
    type="button" 
    id="testUpload" 
    value="Upload" 
    onclick="window.open(
        'http://www.mysite.com/wp-content/themes/mytheme/uploader.php?action=test',
        'popup',
        'width=330,height=235,scrollbars=no,resizable=no,toolbar=no,directories=no,location=no,menubar=no,status=no'
        ); 
    return false" 
/>

I've verified that the uploader.php file is present.

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Can you reach the link by manually entering it into the browser's address bar?

• If not, then you're dealing with problems with the uploader.php (permissions?)
• If yes, there are some Javascript-related problems.