server error:405 - HTTP verb used to access this page is not allowed

Even if you are using IIS or apache, in my guess you are using static html page as a landing page, and by default the web server doesn't allow POST or GET verb on .html page, facebook calls your page via POST/GET verb

the solution would be to rename the page into .php or .aspx and you should be good to go :)

In the Facebook app control panel make sure you have a forward slash on the end of any specified URL if you are only specifying a folder name

i.e.

Page Tab URL: http://mypagetabserver.com/custom_tab/