Facebook had deprecated the support for static FBML on fan page, which now it has to be done in iframe apps method, and add the apps to the fan page tab.
Today is the first time I put create an apps and pointed it to our server, to a simple html file, when the apps was added to the fan page tab, it shows HTTP 405 error, previously had setup a page and hosted on a same server, same directory, and the apps settings were same as the new one, and both were added to the same fan page, but existing apps work just fine.
I’m surprised how easy it is to fix it, I’m not sure what’s the reason behind it, but simply changing the .html to .php, it works like charm.