Facebook page tab - constant 400 error, POST instead of GET

I've added a tab to my facebook page, but all I can see is an 400 bad request error. If I put the tab's URL directly in my browser (or do PPM->Show the frame in browser on FB tab), everything is OK.

Here's the facebook tab I'm talking about: https://www.facebook.com/alphazeroband?v=app_723522437659801 Here's the URL of the page that should be inside FB tab (pure HTML): https://novaer.pl/alphazero/listen.html

And this is something strange I've found in server logs:

"POST /alphazero/listen.html HTTP/1.1" 400 191 "https://s-static.ak.facebook.com/platform/page_proxy.php?v=5"

Shouldn't there be a GET instead of POST?

Solution

I've fixed it by changing HTML into PHP file and adding this in the very beggining on the page:

if ($_POST) {
   header("Location: " . $_SERVER['REQUEST_URI']);
   exit(); 
}

But still I don't know why there was POST instead of GET inside Facebook iframe.