I was creating a facebook app and trying to host the canvas URL on my 1and1 hosting account. Even though I could browse directly to the index.html and it would render fine, I would get an error when trying to browse to it via apps.facebook.com.
“Method Not Allowed. The requested method POST is not allowed for the URL …”
Although I have not encountered this problem on other web hosts, it appears that 1and1’s web server is set up a little differently. When you browse to the canvas page via apps.facebook.com, facebook sends some POST variables to your site. 1and1 doesn’t allow an html file to render when POST variables are sent.
A simple workaround is to rename your index.html to index.php. That makes 1and1 think that the POST variables are expected.